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  • richardmitnick 12:53 pm on January 1, 2021 Permalink | Reply
    Tags: "Terrestrial analogs: A slice of Mars in your own backyard", Astrobites, , , , , , , , , , The study of planetary surfaces is mostly limited to remote investigations using telescopes and robots.   

    From astrobites: “Terrestrial analogs: A slice of Mars in your own backyard” 

    Astrobites bloc

    From astrobites

    Jan 1, 2021
    Anthony Maue
    Edited by: Ashley Piccone

    Striking similarities

    Aside from a few lunar missions you may have heard of, the study of planetary surfaces is mostly limited to remote investigations using telescopes and robots. Although there is some debate over the necessity of sending humans to other worlds, it’s certainly the case that many terrestrial geologists benefit from investigating their study sites in person. Thus, it’s no wonder that some planetary scientists embark into the field all across our planet to study features that may be considered comparable (or analogous) to those on other worlds—features known as terrestrial analogs.

    When a planetary probe returns images of a geologic feature on Mars that is consistent with a known feature on Earth, we can study that analog here on Earth to better understand what geologic processes may have occurred on Mars. Of course, this type of science cannot be as simple as “that channel on Mars looks like rivers on Earth, so it must have formed by water!”. But, at minimum, analog sites are helpful in developing hypotheses for further investigating features on other planets. For example, if you’ve tentatively identified an ancient shoreline on Mars, studying coastal sediments on Earth could tell you what to look for to support your idea with a different dataset, working under the assumption that some aspects of beaches on Mars and Earth would be analogous. Geomorphologist Victor Baker would argue that analogies are in fact the root of all hypotheses in planetary geology.

    Given the nearly airless and frigid conditions of Mars, it makes sense that some of its best terrestrial analog sites are found in Earth’s most extreme environments. Some of the driest places on Earth, like the Atacama Desert in Chile or the McMurdo Dry Valleys in Antarctica, are commonly visited to study geological and biological processes under similarly harsh and Mars-like conditions. Even more accessible places, like the deserts of the southwest United States, can provide the opportunity to study the morphology of rivers in the absence of vegetation, as they would have formed on Mars, for example. To explore whether the putative remnants of hot springs found by the Spirit Rover once held life, we can investigate the different biotic and abiotic processes found at hot springs on Earth that result in similar morphology and geochemistry (see Figure 1). Such a study might take you to geothermal hot spots in Wyoming, Japan, or even Iceland.

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    Figure 1 – Comparison of putative hot spring deposits on Mars (left side) and in the Atacama Desert (right side) which were studied in Ruff and Farmer (2016). Arguments in that paper suggest the similarities in chemistry and texture are consistent with, although do not confirm, a biologic origin from communities of microorganisms common in terrestrial hot springs. White scale bars are 10 cm in (a) and (b) and 5 cm in (c) and (d). Figure 3 from that paper.

    Or maybe not so similar…

    Of course, due to unique conditions on other worlds, many geologic features may have no good Earth analogs! For example, so-called “chaos terrain” found on several planetary bodies (such as Mars and Europa) generally involves a jumbled mess of blocky material that has no real comparison on Earth. Mars also forms bizarre “spider” features when CO2 ice under its surface sublimates and erupts. Another process that likely involves sublimation of volatile ices under non-Earthlike conditions occurs on Mercury to form depressions known as hollows.

    Although planetary exploration continually provides us with some mysterious landscapes never before seen, the diverse terrain of our planet can still provide useful analogs for many other interesting geologic features. In particular, terrestrial analogs are relevant for geologic features on other planets that either formed under conditions similar to the wide range found here on Earth, or else are apparently unperturbed by the significant differences that may exist in composition, gravity, and other properties that vary across the solar system.

    It is also true that terrestrial analogs don’t need to be perfectly equivalent to provide data useful for planetary science (perhaps one is an evil doppelgänger, but not exactly an identical twin). For example, the extensive dune fields around the equator of Saturn’s frigid moon Titan are likely made of organic sand—that is, they’re more like dunes made of coarse coffee grounds than dunes made of the quartz-dominated sand here on Earth. Despite this significant compositional difference, studying the behavior of similarly sized and shaped sand dunes in the Namib Desert of southwest Africa can be useful for making inferences about characteristics and processes we can’t yet observe for distant Titan (see Figure 2). This utility is especially true when combined with other experiments, such as laboratory measurements describing the physical properties of plausible organic sand compositions relative to the properties of quartz sand on Earth. Similarly, studying the blasted rock around the Sudbury Impact Structure in Ontario Canada, despite having been heavily eroded and buried under younger sediments and human development, can help us positively identify impact structures elsewhere in the solar system and infer what properties we should expect from the local geology.

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    The Sudbury and Wanapitei impact craters in Ontario, Canada. Sudbury is the large, elliptical structure (60 x 30 km); Wanapitei is the lake filled crater at upper right. Its diameter is 8km, its age 37 million years. Created with NASA WorldWind by User:Vesta using Landsat 7 (Visible Color) satellite image.

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    Figure 2 – Radar images of dune fields on Titan (left side) compared to images of linear dunes in Middle Eastern and African deserts (right side). PIA15225. Credit: NASA/JPL Photojournal.

    Examples in my neck of the woods

    In northern Arizona, where I currently live, we are fortunate to have a number of excellent field sites that can be considered analogs for planetary environments. For one, the city of Flagstaff sits on top of the San Francisco volcanic field. Essentially every bit of topography in the area is volcanic in origin. At its center is a stratovolcano system that includes the tallest peak in AZ. Nearby Sunset Crater National Monument includes cinder cone volcanoes young enough to have their eruptions vividly described in the stories passed down by local tribes (see summaries on pgs. 79–85). Another famous cinder cone was lovingly named with the initials S.P. (the meaning of which I’ll let you look up and judge for yourself), after its striking appearance. These volcanoes and their relatively young lava flows can be compared to evidence of similarly explosive volcanism preserved on parts of Mars (see Figure 3). Northern AZ is also home to one of the youngest and best-preserved impact craters on Earth, known as Meteor Crater. For anyone remotely interested in astronomy, it’s definitely worth a visit if you’re checking out the nearby Grand Canyon! And for super nerds, the Lunar and Planetary Institute hosts annual training courses in the field, alternating mostly between Meteor and Sudbury Craters, so that planetary scientists can learn to better connect data from planetary probes to real-world features on the ground.

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    Figure 3 – Volcanism local to Flagstaff, AZ compared to some volcanoes on Mars. (A) The San Francisco Peaks are the remnants of a stratovolcano, the tallest of which is Humphreys Peak (Hopi: Aaloosaktukwi; Navajo: Dookʼoʼoosłííd). (B) The Bonito Lava Flow erupted from the base of Sunset Crater, which is the youngest cinder cone of the San Francisco volcanic field and can be seen in the background. (C) SP Crater is a similarly well-preserved cinder cone with a relatively young lava flow. (D) Putative cinder cones and their lava flows on Mars imaged by CTX. Upper images (A) and (B) taken by the author of this bite, lower images (C) from USGS and (D) from Figure 2 of Broz et al (2009).

    In northern AZ, one may also think of the Grand Canyon when they look at Valles Marineris on Mars, but they may be surprised to find that this colossal martian canyon system is at least 5 times deeper than the Grand Canyon, and would span the width of the United States if superposed onto Earth! Of course, features on Mars are generally larger than their earthly counterparts owing to the lower gravity there, but the better analog to Valles Marineris may be massive rift valley systems on Earth, since it is likely tectonic in origin. In contrast, the Grand Canyon formed predominantly through fluvial incision of rivers during uplift of the Colorado Plateau. A simple geomorphological comparison can show how these very different shapes likely indicate different formation processes.

    Still, northern AZ is such a hot spot of planetary science that NASA collaborated with the local United States Geological Survey’s Astrogeology Science Center to prepare Apollo astronauts for geologic structures they might encounter on the Moon. Training sessions occurred in the 1960s and 1970s at Meteor Crater, various local volcanoes, and even a homemade crater field (see Figure 4). Near these sites, one could even round out their southwest space history tour with a visit to nearby Lowell Observatory, the facility where Edwin Hubble discovered the expansion of the universe, Clyde Tombaugh discovered Pluto, and much fantastic astronomical research continues to this day. For the next generation of human exploration, with possible missions to the Moon or Mars in mind, researchers and young astronauts have gone to places like the Utah desert, Hawaiian volcanoes, and the Canary Islands to experience a taste of the desolation and to practice completing objectives on alien landscapes during EVA (extravehicular activity).

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    Figure 4 – Real and artificial craters where astronauts trained near Flagstaff, AZ. (A) 1.2-km-wide Meteor Crater and (B) Dr. Eugene Shoemaker lecturing to Apollo astronauts gathered at its rim. (C) 20-m-wide shallow remains of a crater at Cinder Lake Crater Field manufactured by the Astrogeology Research Center with snowy San Francisco Peaks in the background and (D) Apollo astronauts James Irwin and David Scott testing a prototype lunar rover there back in the day. Images (A) and (C) taken by the author of this bite, images (B) from NAU and (D) from the Forest Service.

    Testing, testing, 1, 2, 3

    Much like the training of the astronauts, terrestrial analogs are also helpful in the preparation of robotic missions to other worlds. Scientists and engineers practice operating instruments and debate sample collection strategies for the Mars Perseverance Rover in the deserts of Nevada.

    Perseverence

    NASA Perseverance Mars Rover.

    Various rovers designed for the Moon and Mars are tested in the high-altitude Atacama Desert to see how they deal with sandy, rocky, and sloped terrains, sometimes with adjusted weights to account for gravity. For the Dragonfly mission to Titan, scientists and engineers will likely test camera systems and software by navigating for landing sites among sand dunes on Earth.

    NASA The Dragonfly mission to Titan.

    Drills and seismometers are deployed on the ice shelves of Alaska, Greenland, and Antarctica, while autonomous submarines explore underneath to see how they might operate on icy ocean worlds like Europa.

    Terrestrial analogs and studies in comparative planetology can improve our understanding of the solar system, and more broadly, our understanding of the general physical processes that may operate throughout the universe. As human and robot missions continue to explore, I think we will continually find that there are important lessons to be learned from the magnificent geology of our home world. Given the vast distances and great costs involved in space exploration, many years may pass before a new mission visits a particular planet (or region) of interest to a planetary scientist. In the meantime, the Earth can sometimes act as a small, but diverse laboratory with which to explore some of the varied terrain found in the cosmos.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 12:45 pm on October 20, 2020 Permalink | Reply
    Tags: "X Marks the Region", Astrobites, , , ,   

    From astrobites: “X Marks the Region” 

    Astrobites bloc

    From astrobites

    1
    Artist’s illustration of a transiting exoplanet. [NASA GSFC]

    Title: Hidden Worlds: Dynamical Architecture Predictions of Undetected Planets in Multi-planet Systems and Applications to TESS Systems
    Authors: Jeremy Dietrich and Dániel Apai
    First Author’s Institution: The University of Arizona, Tucson

    Status: Published in AJ

    Fans and writers of science fiction alike spend countless hours crafting intricate star systems, replete with planets, moons, and a menagerie of space-faring civilisations. The success of missions such as the Kepler Space Telescope (hereafter Kepler) and the Transiting Exoplanet Survey Satellite (TESS) have shown that our solar system is just one of many multi-planet systems present throughout the Milky Way.

    NASA/Kepler Telescope, and K2 March 7, 2009 until November 15, 2018.


    NASA/MIT TESS replaced Kepler in search for exoplanets.

    However, our ability to accurately determine the “planetary architecture” (the orbital configuration of the planets) of a given extrasolar system is severely lacking. Knowing how planets are configured in different extrasolar systems would greatly aid our understanding of how planets form, and how planetary systems evolve (e.g., via planetary migration).

    Exoplanets are inherently difficult to detect, and one of the primary means of detecting them involves measuring transits, the tiny dimming of a star as a planet moves in front of it.

    Planet transit. NASA/Ames.

    To better understand stellar systems, instead of considering each exoplanet individually, we can consider the entire population of exoplanets at once through statistical inference — a method that has only recently become viable thanks to the wealth of data from modern exoplanet surveys. Today’s paper presents a statistical framework — DYNAmical Multi-planet Injection TEster (DYNAMITE) — designed to predict the presence of exoplanets that have so far eluded detection.

    Fire in the Hole!

    The core method at the heart of DYNAMITE is to determine the likelihood of finding an additional planet in an existing multi-planet system, based on the overall statistics of an existing representative population. The authors consider a combined probability density function (PDF) over the inclination, orbital period, and planetary radius, with the key assumption being that each of these parameters has its own independent distribution. Each of these initial PDFs were based on transiting planet data from Kepler, with the range of orbital periods restricted from 0.5 to 730 days, planetary radii from 0.5 to 5 Earth radii, and inclinations between 0 and 180 degrees. Monte Carlo methods (means of approximating something through repeated random sampling) are then used to sample the full probability distributions and “inject” new planets into the system. In order to come up with sensible results, the planetary system must be dynamically stable. This stability depends on the orbits of the innermost and outermost planets, their masses, and the mass of the parent star. It is difficult to accurately determine the masses of exoplanets via the common transit method, so the authors make use of a mass–radius relation to estimate the masses from the planetary radii.

    Sweet Spot

    The model underwent rigorous testing for sensitivity and robustness. Several test scenarios included removing a known planet to see if the model could reproduce it, and removing a planet whilst altering the remaining planets. Figure 1 shows an example of the PDF as a function of orbital period for the Kepler-154 system with the known planet at P = 9.92 days (Kepler-154 f) removed. Of the total Monte Carlo predictions that inject a new planet inside the orbit of the outermost planet, 97% correspond to the region of the removed planet. As for the radius, 67% of the models predictions lie within three standard errors, while the spread is more substantial for the inclination (43%). The mean injections match the known planet’s parameters quite well (as in Figure 1 where the peak is just below the known value for the period), but the authors nevertheless state that since DYNAMITE is primarily aimed at helping guide future observations, it is not designed to provide exact predictions, but rather a likely range of values.

    Speculative Execution

    One of DYNAMITE’s major applications lies in the analysis of systems with candidate planets — planets that are suspected to be there but have not yet been definitively confirmed. TOI 1469 is used as an example to illustrate the iterative nature of the statistical model. Figure 2 shows the various stages of DYNAMITE for the TOI 1469 (HD 219134 / Gliese 892) system. This system is known to have two transiting planets, with at least three non-transiting planets. Starting with only the two known transiting planets, the PDF peaks at around 12.5 days. A planet is inserted here, and the model is run again. Now the PDF peaks near the known planet at around 23 days (HD 219134 f has a period of 22.72 +/- 0.02 days), so we insert another planet here and execute the model again. Proceeding in this manner, the model predicts another planet at ~46 days (corresponding to HD 219134 f with orbital period 46.86 +/- 0.03 days), while in the last iteration the model predicts a fourth planet at ~87 days, corresponding to the unconfirmed candidate planet.

    To Probability Space and Beyond

    Another purpose of DYNAMITE is to analyse the newly identified multi-stellar systems discovered by TESS and identify the systems most likely to contain additional planets so that they can be surveyed again. A sample of known multi-stellar systems from the ExoFOP-TESS archive was tested with the statistical model. Figure 3 shows the overall results of the model using the period ratio model from Kepler, while Figure 4 shows the exact PDF for each TESS system for the orbital period and planetary radius.

    With the ability to predict the locations of hitherto undetected planets, future surveys can be more focused and targeted. Studying these systems in detail, and confirming whether or not these additional planets are present, allows us to constrain and refine models of planetary architectures, our knowledge of the mechanisms that govern the evolution of planetary systems, and, ultimately, our understanding of how exoplanets form.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 12:42 pm on October 17, 2020 Permalink | Reply
    Tags: "Galaxy Formation and Evolution in the Era of the Nancy Grace Roman Telescope", Astrobites, , , , ,   

    From astrobites: “Galaxy Formation and Evolution in the Era of the Nancy Grace Roman Telescope” 

    Astrobites bloc

    From astrobites

    Oct 17, 2020
    John Weaver

    1
    Credit: NASA.

    Conference summary from the “Galaxy Formation and Evolution in the Era of The Nancy Grace Roman Telescope” conference.

    NASA Nancy Grace Roman Space Telescope.

    Astrobite Authors: Lukas Zalesky, Gourav Khullar, & John Weaver.

    Talks were live-streamed on Facebook, and the complete (official) conference proceedings will be available at Zenodo.org shortly.

    What is the Roman Space Telescope?

    You might know it by its original name: the Wide-Field Infrared Space Telescope (WFIRST). Earlier this year the space-based facility was formally named after Nancy Grace Roman, a pioneer in space-based astronomy in an era when women in leadership positions were virtually non-existent. Nancy Grace is known as ‘the mother of Hubble’ (the telescope) and worked tirelessly advocating and organizing the Hubble Space Telescope, which has forever changed our view of the universe.

    Roman was selected as a result of the US Decadal Survey, which highlighted the need for a wide-field infrared space mission to perform survey work in the era post-Hubble and post-Spitzer. It has several distinct science goals all of which are immediately served by the combination of wide-field, infrared imaging and multi-object spectroscopy uniquely enabled by Roman. With a view 100x greater than Hubble in a single snapshot (see Figure 1), broad-band filters, and a slitless grism, along with a groundbreaking coronagraph to directly image exoplanets, Roman is slated to be a key contributor in the next decade of astronomy.

    The event was held as a part of an ongoing series of science meetings to discuss the potential yields of Roman hosted by the Space Telescope Science Institute, attracting nearly 300 participants on the internal event feed and many more on the live stream. This particular meeting was aimed at bringing together the galaxy evolution community to look forward to the era of Roman.

    The first session was focused on outlining the capabilities of Roman, as well as matching those capabilities to the specific science objectives in galaxy evolution that they will enable. The capabilities of Hubble to see deep into the universe with excellent spatial resolution as well as multi-object spectroscopy have transformed our understanding of galaxies, and so it makes sense to see them again in Roman. However, the ‘pencil-beam’ surveys of Hubble, although extremely deep, have been hampered by their size for the simple reason that galaxies obey gravity and form clusters (i.e. the cosmic web). We require a much larger, but still deep, field of galaxies in order to advance our understanding of galaxy evolution, which Roman will do owing to its extremely large field of view.

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    Figure 1: Roman’s incredible field-of-view with Hubble-resolution will immediately dwarf the ultra-deep field which transformed galaxy evolution. Surely Roman will do the same.

    The downside and the upside to all of this extremely deep and incredibly wide imaging are the same: we will have an enormous number of galaxies to study and the amount of data will be insane, on the scale of petabytes. Speakers highlighted the necessity (not recommendation) to begin thinking seriously about how we will interact with such datasets. You won’t be able to download all the images on your home computer, so toolkits and interfaces must be developed to enable the wider community to access the data in meaningful, productive ways. This will be one aspect that will set aside the next decade of astronomy from anything we have previously dealt with.

    In addition to the 10 sessions we briefly summarize below, there were an enormous number of contributions in the form of posters and pre-recorded talks which significantly benefited the overall science discussion of the event. The authors of this bite wish to sincerely thank everyone who attended for making the week so enjoyable and interesting.

    Session 2: Connecting the Near-Field and Deep Field View of Galaxy Formation

    Bullock, Williams, McQuinn, GuhaThakurta, Pearson

    This session focused on investigating local galaxies in spectacular resolution, and how we can use lessons learned in the nearby universe to study the more distant cosmos. Again, we see Romans unique wide field of view take center stage, this time to map out galaxy stellar populations and other properties rapidly in a way that would be far too expensive and time-consuming Hubble.

    Additionally, the need was raised to utilize Roman to explore ultra-faint and diffuse galaxies which until recently were extremely difficult to image owing to their low surface brightness. Enabled by Roman’s wide field of view and extremely sensitive detectors, this kind of science should become commonplace, and will allow us to better understand the role of dark matter as it’s most unexplored domain is in these faint, small galaxies. On a similar note, exploring other faint features of nearby galaxies such as tidal streams and merger debris will be deeply empowered by Roman for these same reasons. Lastly, repeated long-baseline observations can further science in the extremely near-field — our own galaxy — by synergy with Gaia to constrain the proper motion of stars.

    Session 3: High-Redshift Galaxies

    Bowler, Bagley, Wold, Koekemoer, Khullar, Rhoads

    The deep + wide approach enabled by Roman will enable us to locate the first galaxies in the universe, which are extremely faint and so they are incredibly difficult to find. With a greater area of the sky surveyed, we stand a much better chance of finding these lifelines in our understanding of galaxy formation and assembly within the first billion years.

    This session focused on how to locate these first galaxies, or at least the easiest ones to find in the first place: bright, massive galaxies. As it turns out, these kinds of galaxies, although ‘easy to find’ (think Hot Jupiters in exoplanet science), are the exact kinds of galaxies that help to constrain our models of how galaxies grow. The case is simple: it’s easy to grow a small-ish galaxy quickly, but to build a massive galaxy within a billion years is quite a big challenge to simulations and theorists!

    Specifically, the speakers highlighted the need to survey large numbers of these early giants to understand the distribution of their properties and how they contributed to re-ionizing the neutral hydrogen that pervaded the inter-galactic space following the big bang. Helpful probes, such as gravitational lensing (as discussed in-depth by an author of this bite!) and supernovae, can aid us in gaining even more information about these first galaxies.

    Session 4: AGN and Blackholes

    Walsh, Woods, Wang, Petric, Wingyee Lau

    Session 4 detailed the need to understand many of the deeply mysterious aspects of supermassive black holes which still elude us. Despite being awarded as the topic of a Nobel Prize earlier this month, our understanding of these cosmic giants has only just begun.

    Speakers discussed the need to connect the observed properties and behaviors of supermassive black holes and the galaxies which host them (as seen in their quasar-mode), requiring physics to bridge several orders of magnitude in relative size from that of the entire galaxy down to a region the size of our solar system dominated by the supermassive black hole. As Roman is equipped with incredible sensitivity and a wide field, it makes sense to try and find some of the earliest, most massive quasars in the universe, which to date have eluded more narrow searches.

    In particular, the case was made for linking the birth of quasars to the difference in observed properties we see, namely the level of dust obscuration local to the black hole to make a ‘red’ quasar. What causes the build-up of this dust, and how does a galaxy undergo a ‘blow out’ episode to become an unobscured, blue quasar, if ever? Roman will provide new means of finding the dimmest of these dusty quasars in the hopes of better understanding why they exist in the first place, and what fraction of the true quasar/AGN population they represent. Others want to go a step further to ask how did supermassive black holes form in the early universe? Building up that much mass takes a lot of time — more than the universe had at that point!

    Session 5: Data Science for Astrophysics in the Roman Era

    Gawiser, Lower, Gilda, Zanisi, Huertas-Company

    SED fitting and Machine Learning were the main themes for this section. Each speaker (in one way or the other) advocated for using the latest techniques in making forecasts for galaxy morphologies, photometric redshifts, galaxy stellar masses and star formation histories, metallicity evolution, and dust characterization. Moreover, ground-truthing these methods in both real and simulated observations was a theme shared by many advocates in this session.

    Some of the ideas from this session include phasing out functional forms of star formation histories due to biases, using fast and efficient machine/deep learning methods to quantify stellar masses and morphologies in thousands of future-discovered galaxies, and paying attention to systematic uncertainties in both model and data when predicting physical properties of distant galaxies that effectively look like point sources.

    Session 6: Relating the Dark Matter Density Field to Galaxy Properties

    Papovich, Kubo, Huang, Samuel, Chapman

    This session spoke to participants who were interested in making the connection between dark matter and baryonic matter in the Universe, on scales from satellite galaxies to proto-galaxy clusters! Speakers advocated for studying low surface brightness galaxies that highlight the connection of the dark matter halo to mass assembly (and even mass loss), characterizing the nascent predecessors of galaxy clusters with deep surveys by connecting their infrared properties (Roman) to sub-millimeter and radio properties (South Pole Telescope, ALMA).

    South Pole Telescope SPTPOL. The SPT collaboration is made up of over a dozen (mostly North American) institutions, including the University of Chicago; the University of California, Berkeley; Case Western Reserve University; Harvard/Smithsonian Astrophysical Observatory; the University of Colorado Boulder; McGill (CA) University; The University of Illinois at Urbana-Champaign; University of California, Davis; Ludwig Maximilians Universität München (DE), Argonne National Laboratory, and the National Institute for Standards and Technology. It is funded by the National Science Foundation.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

    Moreover, we heard about even answering some questions close to home — what is the distribution of satellite galaxies around the Milky Way?

    3

    Session 7: Dust & Star Formation in Galaxies

    Calzetti, Battisti, Faucher-Giguere, Landt, Alavi

    One of the most fundamental aspects of inferring galaxy properties across cosmic time is figuring out how much of the radiation emission of galaxies has been obscured by dust attenuation. This session was a reminder that dust attenuation models are often assumptions (e.g. a blanket screen vs a spherical dust cloud in the Milky Way), which require strict testing. Speakers in this session spoke of the geometry and chemistry of dust in a diverse set of galaxies and matched those to physical phenomena like star-formation cessation (i.e. quenching) and galactic winds captured in simulations. In synergy with other telescopes, Roman has the capacity to break several logjams surrounding dust observations and their correlations with properties like stellar mass, star formation rate, and metallicity in galaxies.

    Session 8: Challenges for Theory, Modeling, and Synthetic Observations

    Wechsler, Drakos, Weaver, Garg, Ghosh

    Thursday’s second session discussed the needs of the theory, modeling, and simulation communities. Firstly, Rise Wechsler gave one of the only purely theoretical presentations during the conference where she outlined the case for Roman to bring forth a new level of constraints for cosmological-volume galaxy simulations, urging observers to utilize more sophisticated analyses to better match those being provided by simulation teams. Other talks varied in their content greatly but focused mainly on extracting galaxy properties from simulations (including a talk by Prerak Garg) and what we can look forward to with Roman.

    A talk on mock catalogs by Nicole Drakos introduced us to exactly the kind of view we expect with Roman and provides proof of the challenge that will await for photometry techniques when the first real data becomes available. Significant innovation will be required, with a step above what is already the frontier of galaxy photometry highlighted by a talk by John Weaver (also an author of this bite!). This session also saw a foray into machine learning by Aritra Ghosh, which together with mentions in other talks underscores the need for and highlights the interest in machine learning approaches to data analysis in the era of Roman.

    Session 9: Synergies with Wavelengths and Facilities

    Chary, Banerji, Pozzetti, Zalesky, Muzzin, Robertson

    This series of talks began with a discussion of the existing mysteries within galaxy evolution, particularly within the epoch of reionization (z > 6), and the wavelengths at which we can hope to address them. Observations with Roman will contribute enormously to existing and future datasets by providing ultradeep NIR photometry and slitless spectroscopy over enormous regions of the sky. In particular, the combination of Roman’s High Latitude Survey, which will cover some 2,000 square degrees on the sky down to 27th mag, and Rubin providing depth-matched photometry in the optical regime, will provide one of the most exquisite datasets for extragalactic astronomy ever produced. Prior to the launch of Roman, ESA hopes to launch the Euclid satellite, which will survey over 15,000 square degrees to 24th mag, mapping the sky in NIR and laying much of the foundation for deeper observations with Roman. Collectively, these datasets will enable a premier census of galaxy properties across space and time, sampling a range of environments and extragalactic scales.

    The Roman telescope will not launch for another 5 years. Euclid is roughly 2 years from launching, and Rubin‘s primary survey (LSST) will require 10 years after its first light to achieve its final depths. However, some of the exciting science promised by the combination of these surveys can begin sooner, thanks to surveys like the Hawaii Two-0 Twenty Square Degree survey. The Hawaii Two-0 survey will cover 2 of the 3 Euclid Deep Fields with deep optical imaging with Subaru Hyper Suprime-Cam and also benefits from deep Spitzer mid-infrared imaging (both processed by authors of this bite!). Euclid will cover these fields immediately upon its launch, and together with Hawaii Two-0, we will get a glimpse into the future science promised by ultradeep optical+NIR surveys over large cosmic volumes.

    Session 10: Galaxy Formation across Decades of Physical Scales

    Treu, Marchesini, Bell, Rodriguez, Zhang

    As the first speaker of this session made clear in his opening statements, the Roman telescope “will make even the rarest of astronomical objects common.” Rare objects in space tend to reveal unique information about the universe, and the first topic focused on the rare class of objects of strong gravitational lenses. Using strong lenses, astronomers can gain unique insights into the shape of the mass profiles of galaxies, and also learn about the interplay between their dark and baryonic components. In this respect, the Roman telescope will prove enormously fruitful, as it will discover and enable precision characteristics of over 20,000 strong lenses (where 2,000 of these are expected to be spiral galaxies, which make up only ~10% of the known strong-lensing systems).

    Here we also learned of a few other areas where Roman will shine, which include measuring the properties of galaxies across a range of scales. With Roman’s HLS covering over 2,000 square degrees down a K-band magnitude limit of ~27th mag, we will be able to precisely measure the properties of galaxies as a function of galaxy type, morphology, environment, and stellar mass, obtaining high-quality statistics in even the more rare regimes like in high-mass systems. With Roman’s resolution and depth, we will also be able to trace fainter galaxies within groups, ordinarily difficult to detect, to better understand the role of environment in driving galaxy evolution.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:16 pm on August 23, 2020 Permalink | Reply
    Tags: "The First Gamma-Ray Pulsar Confirmed by the People", (MSPs)-millisecond pulsars, 3FGL J2039.6−5618 is indeed a redback MSP now confirmed as PSR J2039-5617., A lesson in gamma-ray pulsar searching, Astrobites, , , , , , Pulsars are rapidly rotating neutron stars that emit radio waves like a light house as they rotate., The gamma-ray source 3FGL J2039.6−5618   

    From astrobites: “The First Gamma-Ray Pulsar Confirmed by the People” 

    Astrobites bloc

    From astrobites

    Title: Einstein@Home Discovery of Gamma-ray Pulsations Confirms the Redback Nature of 3FGL J2039.6-5618

    Authors: C. J. Clark, L. Nieder, G. Voisin, B. Allen, C. Aulbert, O. Behnke, R. P. Breton, C. Choquet, A. Corongiu, V. S. Dhillon, H. B. Eggenstein, H. Fehrmann, L. Guillemot, A. K. Harding, M. R. Kennedy, B. Machenschalk, T. R. Marsh, D. Mata Sánchez, R. P. Mignani, J. Stringer, Z. Wadiasingh, J. Wu

    First Author’s Institution: Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, M13 9PL, UK

    Status: Submitted to MNRAS, open access on arXiv.

    Pulsars are rapidly rotating neutron stars that emit radio waves like a light house as they rotate.

    Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    They come in many flavors, like millisecond pulsars (MSPs), pulsars that complete a rotation in less than 30 milliseconds, many of which belong to a family of “spider” pulsars. These spider pulsars are so named because they blow away or accrete the mass of their binary companion, similar to how some spiders kill their male partners. The two (or three) types of spider pulsars are known as black widows, which have accreted most of the mass from their binary companion star, and redbacks, which are currently accreting mass from their companions.

    Spider MSPs are particularly hard to find when searching in the radio regime due to excess gas from their companions obscuring the pulsed emission. However, since the Fermi Gamma-Ray Space Telescope started publishing catalogs of unassociated gamma-ray sources, many have been found to be spider MSPs in follow up radio observations.

    NASA/Fermi LAT


    NASA/Fermi Gamma Ray Space Telescope

    But today’s paper has, for the first time, confirmed the redback MSP nature of the gamma-ray source 3FGL J2039.6−5618 by detecting gamma-ray pulses before finding the radio pulses!

    A lesson in gamma-ray pulsar searching

    Many MSPs have binary companions, which is what makes searching for their gamma-rays pulses so difficult. Unlike in radio searches, gamma-ray searches need to know the orbital parameters of the binary system extremely accurately. This is because there are so few gamma-ray photons emitted from MSPs that the Doppler shift from the orbital motion of the MSP and its companion will smear out the pulses and make them undetectable. So how did the authors of this paper find these gamma-ray pulses?

    They used observations of 3FGL J2039.6−5618 that had been taken with the XMM-Newton X-ray Observatory , and optical observations with telescopes at the European Southern Observatory, GAIA, and the SOAR telescope, to constrain the orbit.

    ESA/XMM Newton

    These telescopes all found that 3FGL J2039.6−5618 had some kind of optical companion, most likely a main sequence star, and were able to trace out its orbital parameters. This suggested that this gamma-ray source was a redback MSP.

    ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
    •KUEYEN (UT2; The Moon ),
    •MELIPAL (UT3; The Southern Cross ), and
    •YEPUN (UT4; Venus – as evening star).
    elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

    ESA/GAIA satellite


    SART telescope (SOAR) situated on Cerro Pachón, just to the southeast of Cerro Tololo on the AURA site at an altitude of 2,700 meters (8,775 feet) above sea level

    With orbital parameters in hand, the search for gamma-ray pulsations can begin. However, searching for these pulsations is extremely computational difficult, since many properties of the pulsar, such as its spin period, are still unknown. To help with the search, the authors utilized the Einstein@Home, a BOINC volunteer computing system.

    einstein@home

    This system splits up the huge amount of computation into small chunks, and then uses idle volunteer computers to process these small chunks, greatly increasing the speed of the search (you can sign up here if you’re interested). The results of this search found a periodic signal in the gamma-ray data with a 2.65 ms period, and a minimum companion mass of 0.15 solar masses, or a mass 0.15 times that of the Sun, confirming that 3FGL J2039.6−5618 is indeed a redback MSP, now confirmed as PSR J2039-5617.

    A lesson in gamma-ray pulsar timing

    1
    Figure 1: Left: The intensity of the gamma-ray emission as a function of the pulsar spin phase over time. The two clear black lines show the periodic increase in emission that is constant over time, indicating that this source is indeed a pulsar. The initial pulsar timing model parameters were used for this plot, and the wobble in the lines shows that this model may need to be further refined. Right: Same as left, but using the pulsar timing model parameters from the MCMC fitting. The clear straight lines show that this model is a significantly better fit than the original model (Figure 1 in the paper).

    Once the pulsar has been found, its parameters can be refined even more by timing it, a process that minimizes the differences between the model of when the gamma-ray pulses are detected and when the model predicts they will be detected. This can lead to better constraints on the orbital parameters of PSR J2039-5617, which in turn allows us to learn more about its binary companion and the nature of redback systems. For gamma-ray pulsars, this is a complicated process that the authors complete through Markov Chain Monte Carlo (MCMC) sampling. In an MCMC, value of the parameter are randomly chosen according to a predefined distribution (here, based on the initial orbital and pulsar parameters). The likelihood that the randomly chosen values match the data is then computed, and new values are chosen until the most likely values for each parameter are found. While this is a computationally long process, the results are worthwhile, as shown by the improvement in the pulsar binary model in the right panel of Figure 1 compared to the left panel.

    The timing model found using the gamma-ray detection can then be used to look for pulses in the X-ray emission of PSR J2039-5617, which were detected and are clearly shown with the gamma-ray pulses in Figure 2. From this modeling, the authors of today’s paper were able to learn more about PSR J2039-5617 than we have time to discuss here!

    2
    Figure 2: Top: Gamma-ray pulse profile of PSR J2039-5617. The profile is shown twice for clarity, with peaks at an orbital phase of 0.25 and 1.25 clearly visible. The y-axis shows the number of gamma-ray photons detected at each orbital phase, and the red line shows what the expected background gamma-ray count would be if there were no pulsar observed. Bottom: X-ray pulse profile of PSR J2039-5617. Here the number of X-ray photons observed per second as a function of the pulsar’s orbital phase. The peaks around phases of 0.6 and 1.6 clearly show a pulse of X-ray emission (Figure 3 in the paper.)

    Class takeaways

    There are many more unidentified gamma-ray sources from Fermi. If an X-ray or optical companion and orbit can be found, these sources can be searched for pulses and possibly confirmed as MSPs. While this may be just a small number of systems, they may be easier to confirm as pulsars by searching the gamma-rays than the radio observations. Additionally, the discovery of more redback MSPs will help us learn more about pulsar evolution and binary system formation. Timing these gamma-ray pulsars also improve models used to find pulses at other wavelengths. The model for PSR J2039-5617 was even used to detect radio pulses from it, as discussed in this companion paper [MNRAS]. With the first confirmation of a pulsar through gamma-ray pulsations, the future looks bright, literally.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 11:21 am on June 9, 2020 Permalink | Reply
    Tags: "How Do You Weigh a Galaxy?", Astrobites, , , ,   

    From astrobites: “How Do You Weigh a Galaxy?” 

    From astrobites

    9 June 2020
    Haley Wahl

    1
    Artist’s illustration of our galaxy, the Milky Way. [ESA]

    Title: Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
    https://iopscience.iop.org/article/10.3847/1538-4357/ab089f
    Authors: Laura L. Watkins et al.
    First Author’s Institution: University of Chicago

    Status: Published in ApJ

    We can’t put it on a digital scale, we can’t hang it on a balance and compare it against something else, so how does one measure the mass of our home galaxy? The authors of today’s paper use measurements of globular clusters in the halo of the galaxy taken from the Gaia satellite to estimate a mass for the Milky Way.

    What Is Our Galaxy Made of and Why Should We Weigh It?

    Our galaxy contains four major parts: the bulge, the disk (which contains the thin disk and the thick disk), the bar, and the halo (see Figure 1). The first three components are made up of baryons, particles that make up protons and neutrons and therefore most of the things around us. The halo, however, is dominated by dark matter, and the percentage of baryonic mass in the halo depends on how much dark matter there is. Dark matter is a mysterious substance that pervades the galaxy, interacting strongly with gravity and weakly with light. We know dark matter is there because of the rotation curve of the galaxy; if the mass was concentrated at the center, the velocity of the outer regions would be slower than the inner regions. In the case of the Milky Way, we see that the rotational velocity stays fairly constant all the way out, which points to some unseen matter being present (matter that we identify as dark matter). Because of its weak interactions with light, it can be really tough to measure the amount of dark matter, and thus how much it weighs. Overcoming this challenge to calculate a mass for the dark matter in our galaxy’s halo would be a big step in obtaining the mass of the Milky Way.

    Measuring the mass of our galaxy is very useful for two reasons: first, because the mass of the galaxy and its distribution are linked to the formation and growth of our universe. Accurately determining the mass will help us understand where our galaxy sits on the scale of the cosmos. Second, it helps us learn about the dynamical history and future of the Local Group and the satellite population (specifically stellar streams).

    2
    Figure 1: Left: where the Sun sits in the Milky Way, from a face-on perspective. Right: The different parts of the galaxy, from an edge-on perspective. [ESA]

    How to Weigh a Galaxy

    The estimate of the mass of a galaxy is dependent on many things, including which satellites are bound and how long they have been that way, the shape of the Milky Way, and the method used for analysis. Three techniques have been mainly used to measure the mass of the galaxy: the timing argument, abundance-matching studies, and dynamical methods. The timing argument measures the speed at which two galaxies are approaching each other and uses those dynamics to predict a mass. Abundance-matching studies uses the number of galaxies versus their circular velocity and the Tully-Fischer relation to obtain their luminosity, which can be used to estimate their mass. Finally, dynamical methods look at the velocity of tracer objects such as globular clusters; any mass distribution gives rise to a gravitational potential that causes objects to move, so by studying the motions of the objects, we can work backwards to recover the gravitational potential, and thus the mass. The authors of today’s paper use this dynamical method to measure the mass of the Milky Way.

    Using Gaia to Map Motions

    ESA/GAIA satellite

    The team used data from the Gaia mission’s 2nd data release (DR2) to measure the proper motions of stars, or how they are moving across the sky. Gaia is a space-based instrument whose goal is to make a 3D map of the galaxy, and this data release contained measurements for billions of stars and 75 globular clusters. Gaia’s observations are so precise that it can measure a human hair’s width at 1,000 km, which is a resolution 1,000–2,000 times higher than that of the Hubble Space Telescope! (Check out this really cool video on Gaia to learn more about this amazing satellite.) Figure 2 shows just how many sources Gaia has measured. Out of the 75 globular cluster measurements released in DR2, the authors used 34 of them that spanned a range of distances from 2.0 to 21.1 kiloparsecs from the center of the galaxy — which allowed the authors to trace the Milky Way’s mass out to the outer halo.

    3
    Figure 2: A map of the number of sources Gaia measures on a projection of the plane of the galaxy (centered on the galactic center). The lighter the color, the more sources. The two circles in the bottom right are two very small dwarf galaxies that orbit the Milky Way. This figure shows the billions of stars contained in DR2. [Brown et al. 2018]

    In order to map the mass of the galaxy correctly, they need parameters like velocity anisotropy (which measures how the motions of stars vary in different directions), the density of the galaxy, and the potential of the galaxy. The team uses an NFW model, which is a model for how the density is distributed within the galaxy, to describe the potential of the galaxy. The authors then run simulations to determine the radius inside which particles are gravitationally bound to each other (the virial radius) and the mass contained inside the virial radius (the virial mass). By varying the virial parameters and sampling different models of the halo, the team was able to figure out the most probable mass of the galaxy. In addition, they use the velocities of the stars to map the circular velocity of the galaxy out to the radius of the farthest globular cluster. Figure 3 shows the potential of the different components of the galaxy and the results of varying the virial parameters of the halo.

    4
    Figure 3: The potential of the galaxy versus distance. Each component of the galaxy is labeled. The authors vary the virial radius and concentration (which represents the density) of the halo, and the different values they sample over are shown by the shaded region around the halo curve. The combination of the components (i.e., the total potential of the galaxy) is the gray line. The authors map the potential of the entire galaxy, but the vertical dotted lines show the area in which they’re interested, which is the distance of the nearest and farthest globular cluster in their sample. The solid lines show the extent of the best-fitting power law to that region, and the dashed lines show the power-law fit outside the region of interest. [Watkins et al. 2019]

    Evidence for an Intermediate Mass Milky Way

    The authors find that the mass of the galaxy is 0.21 x 1012 solar masses, the circular velocity of the galaxy at the maximum radius they look at (21.1 kpc) is 206 km/s, and the virial radius is 1.28 x 1012 solar masses. This virial mass fits in most with intermediate values found by other studies. The circular velocity measurement the authors made indicates that the velocity is fairly constant in the outer regions, supporting the idea that dark matter is present in our galaxy. Some of the clusters the team used for measurements are on very radial or very tangential orbits, which could have been the result of galactic collisions. If they remove these clusters, the mass and velocity measurements are still within their error bars, showing that these estimates are robust even if there are substructures of globular clusters in the galaxy.

    The amazing wealth of data from the Gaia mission has allowed the team to make one of the most precise estimates of the mass of the galaxy that has ever been achieved. As Gaia continues its mission over the next few years, it will obtain positions and velocities of even more clusters, paving the way for more robust studies of the mass of our galaxy.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:40 am on May 27, 2020 Permalink | Reply
    Tags: "The 'Where’s Waldo?' of Astrochemistry", , Astrobites, , The Missing Molecule-Propadienone (CH2CCO)   

    From astrobites: “The ‘Where’s Waldo?’ of Astrochemistry” 

    26 May 2020

    1
    Searching for molecules in space can sometimes feel like a Where’s Waldo hunt — but finding the missing pieces helps us better understand our universe. [NASA/Jenny Mottar]

    Title: The Case of H2C3O Isomers, Revisited: Solving the Mystery of the Missing Propadienone
    Authors: Christopher N. Shingledecker et al.
    First Author’s Institution: Center for Astrophysics Studies Max Plank Institute for Extraterrestrial Physics & Institute for Theoretical Chemistry at the University of Stuttgart

    MPG Institute for Astrophysics


    Status: Published in ApJ

    Finding and Making Molecules

    Looking for different chemicals in space is a lot like searching for Waldo in the infamous search and find series “Where’s Wally?” Only imagine that the search and find page is light years away from you and all you have is a flashlight.

    3

    As our knowledge and understanding of chemical evolution in space grows, astronomers are seeking the detection of more and more complex organic molecules (COMs). Molecules that could lead to the production of life (like prebiotic molecules that may eventually form DNA) and other larger COMs are rather difficult to detect, so we often use theoretical calculations to predict the evolution and abundance of these larger molecules.

    Chemical models commonly use kinetics, how energy changes over as a reaction progresses, to determine the rate at which chemical reactions occur, and thus the rate at which more complex molecules form and how abundances vary over time. Kinetics tells us that chemical reactions typically have an energy barrier to get from reactants to products. However, space is so cold that there isn’t enough energy available to overcome energy barriers (imagine pushing a 500 pound boulder over the top of Mount Everest). So, we assume that only barrier-less reactions can occur in space. There is a noteworthy exception of ultra hot regions like HII regions, supernovae, and such, where temperatures are high enough to overcome reaction barriers.

    4
    Most chemical reactions must overcome a reaction barrier to get from reactants to products, but most astronomical settings aren’t warm enough to provide the energy necessary to overcome these barriers. [Libretexts]

    One of the most important aspects of theoretical research is matching observational data. If theoretical models using activation barriers and chemical kinetics are not able to match observations, then that usually indicates that there is a physical or chemical process that we don’t know about.

    The Missing Molecule

    In the last decade, one important molecule that has alluded astronomers is CH2CCO, or propadienone. CH2CCO is actually one of three different molecules that can be made from two hydrogen atoms, three carbon atoms, and one oxygen atom (H2C3O). These are known as structural isomers, meaning they’re made up of all the same atoms, but the atoms can be arranged differently to make different molecules.

    5
    The three molecules we can make from H2C3O. Each isomer is made up of the same components, just as the three “Waldo” cartoons above them. However, each H2C3O isomer is put together in a different order, similar to the “Waldo isomers.” Each Waldo is made up of the same colors, but the colors are arranged in different orders.
    [H2C3O isomer structures: Hudson & Gerakines 2019; “Waldo”: Waldo Wiki]

    Propadienone (CH2CCO) is the most stable isomer of H2C3O, meaning CH2CCO has the lowest ground state energy and the H2C3O atoms are “happiest” in the CH2CCO configuration. According the the minimum energy principle, which uses thermodynamics rather than kinetics to predict chemical evolution, CH2CCO should be the most abundant of the three isomers, since it is the most stable of the three. Despite observational efforts and archival data searches, no one has been able to detect CH2CCO in space even though the other two H2C3O isomers have been detected. As the minimum energy principle states that CH2CCO should be detectable as well, this disagreement between observations and theory challenged the minimum energy principle and questioned the validity of relying on kinetics for chemical models.

    Where’s CH2CCO?

    So, where is CH2CCO? As it turns out, we still haven’t detected it in space. However, today’s paper uses theoretical calculations to find “where” CH2CCO is hiding. The authors map reactions associated with the H2C3O isomers using density functional theory (DFT). DFT uses quantum mechanics and kinetics to determine the most stable structures of molecules and their associated energies. CH2CCO can react with two hydrogen atoms to form propenal (CH2CHCHO). The process of adding a single H atom, or a proton, is a common reaction known as hydrogen addition. CH2CCO undergoes two hydrogen additions to form CH2CHCHO, both of which were found to be barrier-less reactions.

    6
    Left: Reaction diagram from today’s paper showing that adding a hydrogen to CH2CCO is a barrier-less reaction, and thus able to occur in space. Right: Hydrogen additions to CH2CCO to form CH2CHCHO. Each reaction adds a single H atom to the carbon chain. Note the black dots are single, unpaired electrons (radicals). [Shingledecker et al. 2019]

    Interestingly enough, hydrogen addition to the second most stable H2C3O isomer, propynal (HCCCHO), is found to have a reaction barrier. Thus propynal is able to persist in molecular clouds, while CH2CCO is converted to CH2CHCHO. These findings are consistent with both previous experimentation and observations of the Sagittarius B2 molecular cloud, where the two less stable H2C3O isomers and CH2CHCHO were detected, but CH2CCO was not.

    Today’s paper shows that the “missing” molecule propadienone (CH2CCO) was never actually missing; it was just masquerading as CH2CHCHO. This discovery is important, since it shows us that kinetic theory and observations of CH2CCO are actually in agreement, rather than disagreement. Additionally, today’s paper confirms the validity of using chemical kinetics and reaction barriers (or lack of barriers) to predict chemical evolution in astronomical settings.

    Sometimes search and finds, like finding molecules in astronomical settings, can be difficult — but ultimately, finding the missing pieces helps us better understand our universe.

    Now that we’ve found CH2CCO, did you find Waldo in the first figure?

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 2:43 pm on April 24, 2020 Permalink | Reply
    Tags: "A day (and night) in the life of an observational astronomer", Astrobites, , , , , Rosanna Tilbrook   

    From astrobites: “A day (and night) in the life of an observational astronomer” 

    Astrobites bloc

    From astrobites

    Apr 24, 2020
    Rosanna Tilbrook

    1
    Here I am at sunset, just outside of my telescope dome! Behind me is Lesedi, another 1.0m telescope. Earth’s shadow, the dark band below the orange and pink of the sunset, is visible above the horizon.

    Despite what you may think, the day to day life of an astronomer probably isn’t much different to anyone else’s. Most of us spend the majority of the day in our offices, tapping away at a computer, with the occasional meeting and a couple of tea breaks. Sure, we work on some pretty crazy stuff- like black holes and exploding stars and the distant but inevitable demise of the universe– but our daily routine is pretty much your average 9 to 5. That is, until we get to go to a telescope.

    It’s important to note that not all astronomers get the opportunity to, or want to, go observing. Some are more interested in taking a theoretical approach, focussing on creating simulations and models to make sense of what we see. They use computers to work out what happens when entire galaxies collide, discover how stars are born, and even create maps of the entire universe. This work is informed by the data collected by telescopes, and in turn, our observations are often guided by the theory. The two go hand in hand, and a lot of astronomers dabble in both.

    As for me, I’m more on the observational side of things. My research involves looking for new planets in our galaxy with a telescope called the Next Generation Transit Survey, or NGTS.

    ESO NGTS an array of twelve 20-centimetre telescopes at Cerro Paranel, 2,635 metres (8,645 ft) above sea level

    NGTS detects planets by monitoring the light of thousands of stars and measuring tiny periodic changes in their brightness. If these changes are of the right size, shape, and duration, we can infer that a planet has transited and is blocking some of the star’s light. However, it is possible that something else may be mimicking the characteristic signatures of a planet detection, so we need to take follow-up data with other telescopes to confirm our discovery. This is where I come in.

    3
    How we detect exoplanets with NGTS. This is called the ‘transit method’ and involves searching for tiny, periodic changes in a star’s brightness which suggest something small and dark- hopefully a planet- has passed in front of it. Image credit:NASA Ames.

    Hi-ho, hi-ho, it’s off to a remote desert plateau we go…

    NGTS is located in Chile- specifically in the Atacama Desert- which is renowned for being one of the best observing sites in the world. It’s also in the back end of nowhere. This is typical for observatories, which are normally built in very remote places to avoid light and air pollution from towns and cities that contaminates the data. Even with crystal clear air, turbulence in the atmosphere still causes problems, making stars appear to wiggle about slightly, or ‘twinkle’. Whilst this can be pretty when you’re stargazing with your friends on a summer night, unfortunately for us astronomers it means a blurry blob on our images! To minimise the amount of turbulent air above us, we have to get high up- which means the ideal location for our observatories is a mountain or plateau.

    As a result of these requirements, my follow-up observations take me on an 18-hour journey (minimum!) from my home in the drizzly UK to the South African Astronomical Observatory, or SAAO, to use the 1.0 metre Elizabeth telescope.

    3
    SAAO

    1 Meter SAAO Telescope in South Africa

    4
    Sunset at SAAO. The 1.0m telescope is at the forefront of the image.

    SAAO is a four or five hour drive out of Cape Town, about fifteen minutes from a small town on the South African karoo called Sutherland. The remoteness of this facility, like many others, means there’s no chance of finding a local hotel or AirBnB to stay in, so the observatory has its own specially built accommodation a short drive downhill from the telescopes. This is my home for a week or two while I collect my data.

    So what’s it actually like using a big telescope half-way across the world?

    The night-to-night life of an astronomer

    The biggest adjustment to life at an observatory is being semi-nocturnal. My day starts around 1pm, when a hot breakfast is served, but cereal is on-hand 24/7 if I sleep in later. It can be hard to stay focussed during a long night at the telescope, so the afternoon is a good time to get on with some work. Sometimes I’ll go for a walk and catch some sun; you don’t get to see much of it when you’re observing! Lunch is a hot meal at about 6pm, when all the astronomers eat together- people come from all over the world to use the telescopes, so you get the opportunity to meet lots of interesting people working on all kinds of cool astronomy.

    After lunch, it’s time to get ready to go to the telescope! SAAO has a handy website containing precise up-to-date weather information for the observatory, so I’ll check that to see if the conditions are clear enough to observe (I can also poke my head out of the window to see if it’s cloudy, but unfortunately I’m not fitted with humidity meters and anemometers). If it looks like it’s going to be a clear night, I’ll pack my backpack with my laptop, notebooks, a spare sweater, a few extra snacks (okay okay, the bag is 90% snacks) and my all-important night lunch. This is a little care package of sandwiches, drinks, and nibbles which is prepared every evening for each astronomer, and serves as your dinner. Trust me, there is nothing better when you’re observing than a 3am cheese toastie and a hot chocolate!

    The drive up the mountain is short but beautiful as the sun sets over the South African karoo. Occasionally I’ll see springbok or dassies on the way up- one time, there was even a lion on the loose by the observatory (but that’s another story).

    Once I get to the telescope I’ll head to my office for the night, which is a small room to the side of the main part of the dome. Although it may sound romantic, using a telescope doesn’t involve me sitting at one end and peering through an eyepiece all night taking notes! Nowadays, the light is collected by a camera, and astronomers sit in a control room with computers and buttons to control the telescope. This ‘warm room’ is usually located a bit away from where the actual instrumentation is housed, so that every time you open the door light doesn’t shine into the telescope and contaminate your data.

    5
    The warm room in the 1.0m telescope at SAAO- my office when I’m observing!

    At the start of the evening I switch the camera on. Even though I won’t be taking data for a while yet, the detector needs time to cool down to a chilly -50 degrees Celcius, which keeps instrument noise to a minimum. While I wait, I have a few minutes to stand outside and watch the glorious African sunset.

    Then, it’s time to get to work.

    The first thing I do when I start my night of observing is take a few images with the telescope shutter closed, as well as some of the blank sky (before the stars appear). It sounds weird, but these ‘bias’ and ‘flat’ frames are really important for calibrating the science images I’m going to take later, as they account for tiny fluctuations in each pixel of the camera. Ignoring these could ruin the precise measurements I need to make.

    Once it’s dark enough, it’s time to take a look at my targets. I’ll move the telescope to point at the right part of the sky and find my star using a finding chart. I also need to find a nearby bright star to use as a ‘guide’, which the telescope uses as a reference to help it to stay pointed at the same place in the sky as the Earth rotates. The guide star is also a useful tool with which to check the atmospheric conditions, like how “twinkly” the stars are, which has an effect on the quality of our data.

    After I’ve found my target and guide star, it’s pretty much a case of setting the exposure time and number of exposures and hitting go! Planet transits usually take a few hours, so I’m able to basically let the telescope do its thing whilst I get on with some work, or, later in the night, watch some TV or a movie (and eat all those snacks). I’ll keep an eye on the weather and guiding to make sure the data is okay; occasionally when the conditions get really awful, I’ll have to guide by hand, which means moving the telescope by tiny increments every few seconds. It’s not the best way of stabilising the telescope and if the weather doesn’t improve it typically means it’s time to call it a night.

    Sometimes, if I’m feeling brave, I’ll step out of the comfort of the telescope dome and into the inky outside world to look at the stars myself. A clear night sky, viewed with your own eyes, is completely breathtaking, and I implore everyone to try stargazing (properly, away from a city or town) at least once. Due to the lack of extra light around you, the Milky Way becomes immediately visible as a river of stars and dust overhead, and as your eyes adjust to the low light the picture only gets more beautiful as more stars become visible. Being in the southern hemisphere, you’ll also notice two fuzzy blobs– they look like clouds- to the side of the Milky Way, which are in fact dwarf galaxies.

    Large Magellanic Cloud. Adrian Pingstone December 2003

    smc

    Small Magellanic Cloud. 10 November 2005. NASA/ESA Hubble and Digitized Sky Survey 2

    Each blob contains billions of stars and are hundreds of thousands of light years away, and yet it feels like you could reach out and touch them. I feel extremely fortunate to be able to visit parts of the world where the night sky is so magnificently clear, and yet, I also find leaving the dome and stepping into the silent, pitch black all alone slightly terrifying, so I don’t go stargazing when I’m observing as often as I should.

    Back in the dome, and back to work. If the night goes smoothly, I’ll be taking data until just before sunrise. As the sky gets lighter, the data quality decreases as the stars start to fade away into the background of the morning sky. I make sure to shut everything down, including closing the shutter and the telescope dome, before packing up and heading out. The drive back down the mountain is a slow one as I’m not allowed to put my headlights on in case other astronomers are still working, so all I can use are my hazard lights until I get closer to the hostel.

    When I finally get back to my room, I’m pretty exhausted. If I’ve had to shut down early due to the weather, I’ll need to fight off sleep a little longer and stay up to keep my body clock in line with my new nocturnal lifestyle. Otherwise, if everything’s gone to plan and I’ve managed a full night at the telescope, I can collapse into bed just as the birds are starting their morning chorus. Either way, if I’ve managed to get some data, I’m a happy astronomer! And if I haven’t, there’s always tomorrow night…

    A final note: experiences come in all shapes and sizes

    Observing can be exhausting, exciting, frustrating, and awe-inspiring all at once. However, not all astronomers will have the same stories and no two observing trips are the same. In this article, I’ve shared my own experiences, but this is one telescope at one observatory looking at one particular thing. There are lots of different types of telescopes and lots of different ways to measure the weird and wonderful things we study in the night sky, so I encourage you to ask other astronomers about their own experiences! Maybe their science has taken them to an observatory in Thailand, or they’ve studied the Universe using invisible light, or had the opportunity to sit at the helm of one of the largest telescopes on Earth.

    Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, Spain, sited on a volcanic peak 2,267 metres (7,438 ft) above sea level

    And who knows, maybe one day you’ll get to have an observing experience of your own.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:51 am on April 22, 2020 Permalink | Reply
    Tags: "How to Grow a Giant Galaxy", Astrobites, , , ,   

    From astrobites: “How to Grow a Giant Galaxy” 

    Astrobites bloc

    From astrobites

    1
    A projection of stellar matter across a 50 Mpc region of IllustrisTNG100 at present day. These are not individual stars, but groups of stars large enough to be seen. Each clump is most likely a separate galaxy. [TNG Collaboration]

    Title: MOSEL Survey: Tracking the Growth of Massive Galaxies at 2 < z < 4 using Kinematics and the IllustrisTNG Simulation
    Authors: Anshu Gupta et al.
    First Author’s Institution: University of New South Wales, Australia

    Status: Published in ApJ

    How, exactly, galaxies form is still very much an open question in astrophysics. It’s not like we can watch a galaxy evolve — most are about 12 billion years old, and even the youngest we’ve discovered is about 500,000 million years old.

    There are two ways to work around this problem. The first is a simple matter of looking back into time. Light takes a finite amount of time to travel to us, and so the farther away we look, the older that light is. That means that the farther a galaxy is, the younger we see it. Instead of watching a single galaxy evolve over time, we can compare farther (“younger”) galaxies to closer (“older”) galaxies, and interpolate what may have happened to cause any changes.

    The second way to work around our observational conundrum is to watch galaxies evolve in simulation space. The authors of today’s paper used IllustrisTNG100, part of a suite of large cosmological simulations of galaxy evolution. The cover image above shows a subset of luminous matter in the TNG100 simulation.

    Observed Mass, Movement and Star Formation History

    The kinematic properties (how things are moving) of star-forming galaxies is strongly linked to how they gained their mass. Today’s authors compared the velocity dispersion of observed “younger” galaxies at redshift z = 3.0–3.8 to “older” galaxies from previous studies of redshift z ~ 2 and found that their most massive galaxies had smaller velocity dispersions than massive “older” galaxies.

    3
    Figure 1: Velocity dispersion as a function of mass, shown on a log–log scale. The authors’ “younger” galaxies are shown as gold stars. Other shapes represent previous studies of “older” galaxies at z ~ 2. The more massive galaxies in the authors’ sample are represented by larger stars and have smaller velocity dispersions than “older” galaxies of the same mass (shown in the red circle). [Adapted from Gupta et al. 2020]

    By looking at the spectra of these galaxies, the authors could also extract their star formation histories. Basically, this looks at how old current stars are to extract the star formation rate over time. The top panels of Figure 2 show the authors’ results (keep in mind, time reads as newer on the left and older on the right). The bottom two panels show results from previous studies of galaxies at z ~ 2. While the less massive galaxies in the authors’ survey (top left panel) show the same pattern of increasing star formation rate, the most massive galaxies on the right have relatively flat star formation histories. This is in contrast to massive galaxies at z ~ 2, which show an increasing star formation rate over time.

    4
    Figure 2: Star formation histories for four different populations of galaxies. The x-axis is galaxy time before observation and the y-axis is star formation rate. The top two panels are the galaxies from the authors’ survey at z ~ 3 and the bottom two are from a previous survey at z ~ 2. The less massive galaxies are on the left and the more massive galaxies are on the right. The shaded areas indicate errors and the red arrows point toward trends. [Gupta et al. 2020]

    Both the odd star formation histories and velocity dispersions point to something happening between z = 3 and z = 2 that changed massive galaxies. To determine what that might be, the authors turn to simulations.

    Into the Simulation

    The IllustrisTNG100 simulation starts with a distribution of mass at a redshift of z = 127 and runs until present day, z = 0. As it runs, the random fluctuations in density at z = 127 turn into galaxies, which grow, form stars and merge. The authors wanted to look at how these galaxies acquired their stars over time.

    There are basically two ways that a galaxy can gain stars: either by forming them from gas belonging to the galaxy (in situ) or by accreting the stars from other, mostly smaller, galaxies (ex situ). Figure 3 shows the fraction of stellar mass in the simulation that was accreted ex situ, rather than formed in the galaxy. It shows that for the most massive galaxies (in red), the fraction of ex situ stellar mass increases between z = 3 (pink dotted line) and z = 2 (black dotted line). Meanwhile, the ex situ stellar mass fraction remained largely constant for less massive galaxies (blue).

    5
    Figure 3: The fraction of a galaxy’s stellar mass that was obtained from other galaxies, rather than formed in situ. Massive galaxies are shown in red, while less massive galaxies are shown in blue. The salmon and blue shaded regions are, respectively the error for the more massive and least massive galaxies. The black and pink dotted lines indicate, respectively, z = 2 and z = 3. [Gupta et al. 2020]

    Uniting Simulations and Observations

    The authors speculate that this increase in ex situ stellar mass fraction seen in simulations may be responsible for the increase in velocity dispersion seen in observed massive galaxies between z = 3 and z = 2. Turbulence and gravitational instabilities driven by accretion of stars and gas would increase the randomness of velocities (i.e. the velocity dispersion).

    This could also explain the difference in star formation history between massive galaxies at z = 2 and z = 3 (Figure 2). Gas is necessary for the formation of stars and if the galaxies at z = 2 have been able to gain gas from accretion, they would be able to increase their star formation rate, as seen in the bottom right panel of Figure 2. In contrast, a smaller ex situ stellar mass fraction for z = 3 galaxies indicates that there has been less accretion and less opportunity to gain new gas and thus form new stars, leading to the flat star formation trend seen in the top right panel of Figure 2.

    Essentially, the younger galaxies at z = 3 have had less time to merge with other galaxies, leading to smaller velocity dispersions and less star formation.

    The authors note that their conclusions are limited by many factors, including a small sample size. However, these are promising results and show how much can be gained by comparing observations and simulations.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:32 am on April 15, 2020 Permalink | Reply
    Tags: Astrobites, , , , , , ,   

    From astrobites: “How It’s Made, Fast Radio Burst Edition” 

    Astrobites bloc

    From astrobites

    1
    Artist’s conception of the localization of a fast radio burst to its host galaxy. [Danielle Futselaar]

    Title: Spectropolarimetric analysis of FRB 181112 at microsecond resolution: Implications for Fast Radio Burst emission mechanism
    Authors: Hyerin Cho et al.
    First Author’s Institution: Gwangju Institute of Science and Technology, Korea
    Status: Published in ApJL

    Fast radio bursts (FRBs) are probably the fastest growing and most interesting field in radio astronomy right now. These extragalactic, incredibly energetic bursts last just a few milliseconds and come in two flavors, singular and repeating. Recently the number of known FRBs has exploded, as the ​Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope has discovered about 20 repeating FRBs (and also redetected the famous FRB 121102) and over 700 single bursts (hinted at here).

    CHIME Canadian Hydrogen Intensity Mapping Experiment -A partnership between the University of British Columbia, the University of Toronto, McGill University, Yale and the National Research Council in British Columbia, at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia, CA Altitude 545 m (1,788 ft)

    However, despite the huge growth in the known FRB population, we still don’t know what the source(s) of these bursts is (are). Today’s paper looks at possible explanations for the properties of one FRB in particular to try to figure out what its source might be.

    Your Friendly Neighborhood FRB

    A number of previous astrobites have discussed the basics of FRBs (here, here, and here for example) but the FRB that the authors of this paper focus on is FRB 181112. FRB 181112 was found with the Australian Square Kilometer Array Pathfinder (ASKAP) and localized to a host galaxy about 2.7 Gpc away from us even though it has not been observed to repeat.

    Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Australian Mid West. ASKAP consists of 36 identical parabolic antennas, each 12 metres in diameter, working together as a single instrument with a total collecting area of approximately 4,000 square metres.

    That’s over a hundred times farther away than the closest galaxy cluster, the Virgo Cluster!

    Virgo Supercluster NASA


    Virgo Supercluster, Wikipedia

    One quality of FRB 181112 that makes it particularly interesting to study is that the way ASKAP records data allows the authors to study the polarization of the radio emission. Polarization of light is a measure of how much the electromagnetic wave (here the radio emission) rotates due to any magnetic fields it propagates through. The two types of polarization are linear polarization (Q for vertical/horizontal, or V for ±45°), which occurs if the electromagnetic wave rotates in a plane, and circular (either left- or right-handed depending on the rotation direction) if the light rotates on a circular path. By looking at the polarization of FRB 181112, shown in Figure 1, the authors can determine the strength of the magnetic field it traveled through.

    2
    Figure 1: a) The full polarization profile of FRB 181112 showing four profile components. The black line, I, is the sum of all the polarizations of light, or the total intensity of the burst. The red line, Q, is the profile using only (linearly) horizontally or vertically polarized light; the green line, U, is using only the (linearly) ±45° polarized light; and the blue line, V, is the profile using only circularly polarized light. Negative values describe the direction of the polarization. b) The polarization position angle of the zoomed in profiles from panel (a) seen in panel (c). Variation here suggests the emission is coming from different places in the source. d) A three second time series of the data where the FRB is clearly visible at about 1.8 seconds. [Cho et al. 2020]

    In addition to polarization, the dispersion measure (DM), or difference in time of arrival of the FRB at the telescope between the highest and lowest radio emission frequencies due to its journey through the interstellar medium (ISM), can provide information about the properties of the environment(s) the burst travels through. Each of the four components of FRB 181112 (visible in panel (a) of Figure 1 in three different polarizations, Q, U, and V, as well as total intensity, I) are shown in the bottom row of Figure 2, and each component has a slightly different DM. By looking at how the DM changes, the authors can not only look at different emission processes that could lead these apparent changes, but can also measure how scattered the radio emission of FRB 181112 might be due to the ISM. The intensity of the emission as a function of time and radio frequency for each of the four polarization profiles (I , Q, U, and V) are shown in the top row of Figure 2. The four different components that make up FRB 181112 are shown in total intensity, I, in the bottom row of Figure 2.

    3
    Figure 2: Top row: Intensity of the radio emission of each of the four polarization profiles, I, Q, U, and V (described in Figure 1) as a function of time and radio frequency. Bottom row: Close up of the four different pulse components of the total intensity polarization profile, I, of FRB 181112 as a function of time and radio frequency. All components have been assumed to have a DM of 589.265 pc cm-3 , and a slight slope in the intensity as a function of time and frequency can be seen in pulse 4, indicating it may have a slightly different DM. [Cho et al. 2020]

    Properties of FRB 181112

    4
    Figure 3: Degree of polarization of FRB 181112. The black line (P/I) shows the total polarization, the red line (L/I) shows the linear polarization, and the blue line (V/I) shows the circular polarization. The red and black lines show a large amount of polarization constant in time, while the blue line shows the circular polarization changes over the pulse. [Cho et al. 2020]

    The authors first find that FRB 181112 is highly polarized (see Figures 1 and 3), and while the degree of both the total (P/I) and linear (L/I) polarization is constant across all four components of the pulse, the degree of circular (V/I) polarization varies, as shown in Figure 3. This indicates that the FRB must have either traveled through a relativistic plasma, a cold plasma in the ISM that is moving at relativistic speeds, or that the emission was already highly polarized at the time it was emitted, meaning the source of FRB 181112 would have to be highly magnetized. However if the source of the polarization is due to the plasma in the ISM, the expected polarization would be almost completely linear (Q or U), whereas we observe significant circular polarization (V).

    The authors next analyzed the four different components shown in the bottom row of Figure 2 for variations in DM and find there are some small, but significant differences between each component. These differences could be due to some unmodeled structure in the ISM, again possibly a relativistic plasma, but is unlikely since the burst lasts for only 2 milliseconds. The authors also suggest these differences in DM could be due to gravitational lensing, the radio light being bent around a massive object.

    Gravitational Lensing

    Gravitational Lensing NASA/ESA

    This would mean different components travel through different paths in the ISM, accounting for the different DMs and four different components. However, gravitational lensing cannot explain the high degree of polarization seen in FRB 181112.

    The Million Dollar Question

    So how was FRB 181112 made? What caused the polarization and differences in DM? Well, the authors can’t say anything for certain. They suggest that the most likely model is a relativistic plasma close to the source of the emission, which has polarization properties similar to known magnetars (highly magnetized neutron stars known to emit radio bursts), but none of their models can fully explain all of the different properties of FRB 181112. The source of FRB 181112 remains a mystery for now, but with the huge number of FRBs now being detected, the answer may lie just around the corner.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:34 am on March 18, 2020 Permalink | Reply
    Tags: "An Iced Cosmic-Ray Macchiato", Astrobites, , , , ,   

    From astrobites: “An Iced Cosmic-Ray Macchiato” 

    Astrobites bloc

    From astrobites

    1
    Artist’s impression of the shower of particles caused when a cosmic ray, a charged particle often produced by a distant astrophysical source, hits Earth’s upper atmosphere. [J. Yang/NSF]

    Title: Bottom-up Acceleration of Ultra-High-Energy Cosmic Rays in the Jets of Active Galactic Nuclei
    Authors: Rostom Mbarek and Damiano Caprioli
    First Author’s Institution: University of Chicago

    Status: Published in ApJ

    Our universe is littered with particles of unbelievably high energy, called cosmic rays. The most extreme of these particles carry the same amount of energy as a professional tennis serve, like the Oh-My-God Particle detected nearly 30 years ago. The catch: we don’t know exactly what processes can pack so much energy into a single particle. The authors of today’s article discuss how these particles might gain their energy in a way analogous to your morning trip to Dunkin’™.

    Cosmic Rays at a Glance

    Cosmic rays are atomic nuclei that have been accelerated to high energies in astrophysical environments, such as supernova remnants or active galactic nuclei. Although they might seem like a great tool in the multi-messenger astronomy toolbox, astronomy with cosmic rays is no simple task, as these particles get deflected by extragalactic magnetic fields.

    2
    Cosmic rays (red) consist of individual protons and nuclei of heavier elements. They are deflected by magnetic fields along their cosmological odysseys and can’t be used to point back to the place of their origin. [IceCube Neutrino Observatory]

    Despite efforts to pinpoint the origins of cosmic rays, especially those of the highest energies, we’ve come up empty-handed (check out these bites for previous studies: Galactic cosmic rays, cosmic-ray anisotropy).

    Even though we can’t measure where they come from, we do know their energies, and a variety of cosmic-ray experiments detect millions of these particles every year. Many of them are thousands to millions of times more energetic than the particles in the largest terrestrial particle accelerator, the Large Hadron Collider, but we don’t know how the highest energy cosmic rays get their energy.

    Cosmic-Ray Acceleration: Old News

    Many theories of cosmic-ray acceleration tend to revolve around the idea of Fermi acceleration. In this scenario, objects such as supernova remnants can create shocks, consisting of material moving together with supersonic speeds, and these shocks can accelerate particles to high energies. As a shock wave propagates, particles bounce back and forth across the shock boundary. Over time, successive bounces across the shock front lead to a net transfer of energy to the particles.

    While Fermi acceleration does a good job of explaining cosmic rays with moderate energies and has been a staple of models for decades, it has a few pitfalls, and many argue that it can’t provide the whole story for cosmic-ray acceleration at the highest energies.

    A Cosmic Cup o’ Joe

    The authors of today’s paper propose a new way of looking at cosmic-ray acceleration: the espresso mechanism. Why espresso? Because instead of gradually gaining energy over time, particles gain their energy from a single shot.

    3
    In the “espresso mechanism”, particles gain tremendous amounts of energy from entering a jet for a short period of time. Here, a particle with initial momentum and energy pi, Ei enters a jet with characteristic Lorentz factor Γ and leaves the jet with an energy equal to roughly Γ2Ei. [Caprioli et al. 2018]

    Consider an object with a jet, such as an active galaxy. If a low-energy cosmic ray enters the jet (or steam), then it can be shot down the barrel of the jet and get kicked out at much higher energy. In many cases, particle energies can increase by a factor Γ2, where Γ is the Lorentz factor (this reflects how fast the jet is moving). For some jets, this means particles can exit nearly 1,000 times as energetic as they were when they entered the jet.

    4
    In realistically modeled jets, material tends to clump in some regions, and these regions of overdensity (color scale in figure) cause the jet to locally move faster or slower. [Mbarek & Caprioli 2019]

    While this espresso scheme sounds great in principle, many previous calculations have relied on spherical cow treatments of jets, when in reality they are remarkably dynamic and complex structures.

    That’s where the authors of today’s paper come into play. These authors take a simple treatment of the espresso mechanism and complexify it by performing a full magnetohydrodynamic (MHD) simulation of ultrarelativistic jets. This takes factors like small-scale fluctuations of jet speed and jet density into account, to give a more accurate picture of the dynamics of jets.

    By simulating the full structure of jets, the authors find that complex environments don’t weaken the promises of espresso acceleration. In fact, the very imperfections that manifest in realistic jets can help with particle acceleration. What’s more, jet perturbations allow particles to receive double or even triple shots of energy.

    Throughout the paper, the authors describe the emergent spectra of espresso-accelerated cosmic rays. In doing this, they find that espresso acceleration is consistent with current measurements of ultra-high-energy cosmic rays in terms of energy, chemical composition, and spatial distributions, an accomplishment which no other model of cosmic-ray acceleration can boast.

    6

    Sample particle trajectories (black curves) are overlaid on top of slices of the jet, with jet velocity represented by the color in the top panels. Bottom panels show the amount of energy gained along the particle paths, showing that particles can leave jets with much more energy than they entered with. [Mbarek & Caprioli 2019]

    In light of all of this, it’s probably safe to say that the future of cosmic-ray science will be very caffeinated.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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