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  • richardmitnick 9:13 am on January 22, 2020 Permalink | Reply
    Tags: , Astrobites, , , ,   

    From astrobites: “Modeling Supernovae Feedback as a Galactic Fountain” 

    Astrobites bloc

    From astrobites

    Title: How do Supernovae Impact the Circumgalactic Medium? I. Large-Scale Fountains in a Milky Way-Like Galaxy
    Authors: Miao Li and Stephanie Tonnesen
    First Author’s Institution: Center for Computational Astrophysics, Flatiron Institute

    Status: Submitted to ApJ

    1
    Artist’s impression of the galactic fountain model, in which gas expelled from a galaxy rains back down on it. [NRAO/AUI/NSF; Dana Berry/SkyWorks; ALMA (ESO/NAOJ/NRAO)]

    NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

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

    Simulations are a powerful tool in astronomy. Processes that take billions of years in reality can be played out over a much smaller timescale on a computer. The exact time a simulation takes to run depends on a few things, mainly the size and resolution of the simulation, as well as the speed of the computer itself.

    The Problem

    2
    Artist’s impression of a supernova explosion. [ESO/M. Kornmesser]

    Resolution is a critical issue when it comes to any simulation; galaxy formation in particular involves both very large and very small processes. In the early years of galaxy formation simulations, researchers found that simulated galaxies weren’t accurately reproducing the observed rates of star formation. They realized that a relatively small process must be having a large effect, known as feedback. There are two main drivers of feedback: active galactic nuclei and supernovae. Each of these events take place on relatively small physical scales but can change galaxies in ways we are still uncovering.

    The trick comes in simulating these small-scale processes in large simulations. Instead of modeling the actual processes, rough approximations are made and inserted into the simulation. While these approximations are becoming increasingly more realistic, they are still not ideal. The authors of today’s paper, Li and Tonnesen, wanted to investigate how the circumgalactic medium (all the gas and dust surrounding a galaxy) could be affected by supernovae.

    The Process

    Supernovae are the spectacular deaths of large stars. As they die, they throw out large amounts of matter at incredible speeds. Although a single star is relatively small compared to a galaxy, stars in clusters tend to die at around the same time, and the collective energy released can create an outflow that pushes gas out of the galaxy.

    Li and Tonnesen first simulated supernovae to understand these outflows and then inserted the results of that simulation at random into a larger simulation of a galaxy.

    The Product

    The authors found that these outflows created a hot atmosphere surrounding the galaxy which, once fully formed, had fountain modes. Fountain modes are exactly what they sound like — gas closer to the galaxy gets pushed away by the supernovae winds until it gets far enough away that gravity has a stronger effect than the wind and it falls back toward the galaxy.

    3
    Figure 1: A slice through the center of the galaxy in the y–z plane, where y is the horizontal direction and z is vertical. The slices are 400 by 400 kiloparsecs. The different boxes represent number density, temperature, pressure, z-velocity, metallicity and entropy. [Li & Tonnesen 2019]

    Figure 1 shows a cross section of the simulated galaxy and the values of certain parameters. The fountain modes are best depicted in the lower left panel. The plot is a slice from the y–z plane, where z is the vertical axis. The colors represent the velocity in the z direction, red for positive and blue for negative. The mix of red and blue shows that there is not uniform motion away or toward the galaxy, but a constant motion like that of a fountain.

    Much like in Earth’s atmosphere, clouds can form in the galactic atmosphere of gas surrounding the simulated galaxy. As the gas condenses, it sinks back toward the galaxy. We see evidence of high-velocity clouds such as these in the Milky Way.

    4
    The radio emission from high velocity clouds superimposed on a visible light image of the Milky Way. It is clear from this image that these infalling clouds are located well away from the Galactic plane.
    Credit: NASA, B. Wakker, I. Kallick

    The Prospects

    These results paint a fascinating picture of a dynamic galactic atmosphere, but the article only covered the case where the gas driven by the supernovae outflows did not have enough energy to escape the gravitational pull of the galaxy. This resulted in the gas falling back down, creating the fountains. In a future paper, the authors will explore the case where this is not always true, and gas is allowed to leave the galaxy altogether.

    By fully understanding the effects supernovae can have on a single galaxy, better approximations of feedback can be implemented in even larger simulations, which can teach us even more about the universe we inhabit.

    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 2:51 pm on December 30, 2019 Permalink | Reply
    Tags: "A Pulsar’s Surface Map Gets a NICER Update", Astrobites, , , ,   

    From astrobites: “A Pulsar’s Surface Map Gets a NICER Update” 

    Astrobites bloc

    From astrobites

    Dec 30, 2019
    Kaitlyn Shin
    Title: A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
    Authors: Thomas E. Riley, Anna L. Watts, Slavko Bogdanov, et al.
    First Author’s Institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1090GE Amsterdam, The Netherlands
    1
    Status: Appears in an ApJ Letters Focus Issue on NICER Constraints on the Dense Matter Equation of State [open access on the arXiv]

    NASA/NICER on the ISS

    Women in STEM – Dame Susan Jocelyn Bell Burnell

    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.

    Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

    Dame Susan Jocelyn Bell Burnell 2009

    Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

    What do pulsars’ magnetic fields look like?
    Pulsars are rapidly rotating neutron stars with short and regular rotational periods, observed via their beams of electromagnetic radiation. These observed pulses are analogous to the light flashing on a lighthouse where observers can only see the light pulses once every rotation, when the beams are aligned along the line of sight. For rotation-powered pulsars, these light pulses are powered by the neutron stars’ rapid rotations and extremely strong magnetic fields; the canonical (standard) depiction of the magnetic fields that give rise to these pulses can be seen below in Figure 1.

    2
    Figure 1. A textbook view of the magnetic field configuration around a pulsar, where the large-scale external magnetic field has a dipole configuration centered at the neutron star. The magnetic axis is not aligned with the rotation axis and thus electromagnetic beams (in this figure, radio beams) can be observed as pulses with regular rotational periods. The light cylinder, outlined in dashed green, is the radius out to which the co-rotation speed of the magnetic field approaches the speed of light. Credits: Handbook of Pulsar Astronomy by Duncan Ross Lorimer and Michael Kramer (https://www.cv.nrao.edu/course/astr534/Pulsars.html)

    This “textbook view” of a pulsar’s magnetic field depends largely upon the simplifying assumption that the large-scale external magnetic field has a dipole configuration centered at the neutron star. Many of these dipole configuration models give rise to heated spots on the neutron star, where the hot regions are located on opposite poles; X-rays are emitted from these hot regions as the pulsar rotates. However, these dipole models are far from comprehensive, and understanding the mechanisms of pulsar emission is still an active area of research. Indeed, there is observational evidence for higher-order multipole moments (e.g. quadrupoles, octupoles, etc) present near the surface of neutron stars. However, the combination of limited computational power (required for numerically simulating these more complex magnetic fields), and no obvious choice of which alternate magnetic field configurations would better explain observed pulsar emission, makes it difficult to study models more complex than the dipole models.

    A NICER development
    The Neutron star Interior Composition ExploreR (NICER) is a NASA telescope installed aboard the International Space Station in 2017. With its high time resolution and unprecedented sensitivity in the soft X-ray band (0.2–12 keV), NICER has obtained some of the highest-quality pulse profiles (shape of observed pulses) of known X-ray pulsars. Today’s paper focuses on NICER data of one such pulsar, PSR J0030+0451. This pulsar is an isolated pulsar as well as a millisecond pulsar (since its rotation period is ~4.87 ms). It is one of the nearest observed millisecond pulsars, with a relatively well-constrained distance of 329 ± 9 parsecs (about 1070 ± 30 light years) away from Earth.

    In this paper, the authors investigated models that would best explain the observed pulse profile of PSR J0030+0451. Since two distinctly occurring pulse components were observed, the tested models included two distinct hot regions on the surface of the neutron star, with shapes such as circular spots, ring-shaped regions, and crescents. Also tested was the standard configuration of identical hot regions on opposite poles of the neutron star, as well as more complex configurations where the two hot regions were independent and didn’t have to be located at opposite poles. The propagation of the radiation toward the observer (reconstructing the emission from the hot regions) and the instrumental response (the sensitivity of the instrument) were also included in the models.

    Multiple methods were used to determine which physical configuration was optimal, including (but not limited to) evaluating the model evidence (also known as the marginal likelihood), i.e. how well the model answers the question “given a model, how likely is it that the observed data could have come from this model?” After exploring various different models, the authors found that the physical configuration which seemed to best explain the observed data had hot regions modeled as a small circular spot and an extended thin crescent, both located in the same hemisphere of PSR J0030+0451 (Figure 2).

    3
    Figure 2. A view of the inferred hot regions on the surface of PSR J0030+0451, aligned with the equator and viewed at three different positions in the rotation. In this model, the inferred hot regions are the small circular spot and the extended thin crescent, and they have approximately the same effective temperature. (Figure 17 in today’s paper.)

    Today’s study found that the canonical “textbook view” of pulsar magnetic field configurations, where the resulting hot regions are located on opposite poles of the neutron star, was a strongly disfavored model for PSR J0030+0451. A comparison between hot regions from a canonical magnetic field configuration (using gamma-ray and radio pulses), and from the optimal configuration found in today’s paper, can be seen in Figure 3. This extraordinary finding—where the configuration of the pulsar’s hot regions is much more complex than standard models predict—agrees with another independent study of NICER data of PSR J0030+0451, which found a strikingly similar result. For an animated visualization of the inferred hot regions from both studies, check out the embedded video below provided by NASA!

    4
    Figure 3. Another visualization of the hot regions from PSR J0030+0451. The color blue depicts hot regions at opposite poles of the neutron star inferred from a previous study (Johnson et al. 2014), whereas the color red depicts hot regions in the same hemisphere inferred from today’s paper. (Figure 2 in Bilous et al. 2019, an accompanying paper to today’s paper.)


    Video 1. A surface map of pulsar J0030+0451 using X-ray data from NICER. The inferred locations and shapes of the hot regions on the neutron star surface can be found at 2:45. Credits: NASA Goddard (https://youtu.be/zukBXehGHas)

    Astrophysical implications

    The strange configuration of hot regions on a pulsar would have to have been created by a strange magnetic field configuration. Such a complex magnetic field configuration would have implications for studying pulsar emission mechanisms, such as perhaps modifying the interpretation of multi-wavelength emission from pulsars. Additionally, with the optimal model from today’s paper, the authors are able to provide an estimate of the mass and radius of the neutron star of PSR J0030+0451 (approximately 1.34 +0.15/-0.16 solar masses and 12.71 +1.14/-1.19 km, respectively). Constraints on the mass and radius of a neutron star are invaluable for studying its internal composition and the dense matter equation of state, which is a long-enduring astrophysical mystery. With the NICER mission, it is possible that the dense matter equation of state can finally be understood!

    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 12:39 pm on December 27, 2019 Permalink | Reply
    Tags: "TESS reveals HD118203 b transits after 13 years", Astrobites, , , , , Exoplanet HD118203 b   

    From astrobites: “TESS reveals HD118203 b transits after 13 years” 

    Astrobites bloc

    From astrobites

    Dec 26, 2019
    Emma Foxell

    Title:TESS Reveals HD 118203 b to be a Transiting Planet
    https://arxiv.org/abs/1911.05150
    Authors: Joshua Pepper, Stephen R. Kane, Joseph E. Rodriguez et al
    First Author’s Institution: Department of Physics, Lehigh University, Bethlehem, USA

    Status: Submitted

    There are multiple ways to discover an exoplanet. The first exoplanet around a solar type star was discovered by radial velocity measurements, and earned the discoverers’ this year’s Nobel Prize.

    Radial Velocity Method-Las Cumbres Observatory


    Radial velocity Image via SuperWasp http http://www.superwasp.org-exoplanets.htm

    After the advent of wide field exoplanet surveys, from SuperWASP starting in 2006, to NGTS and TESS, most exoplanets have been discovered using the transit method and confirmed by radial velocity.

    Planet transit. NASA/Ames

    However, the exoplanet in today’s article, HD118203 b, was detected by radial velocity back in 2006 and has been found to transit 13 years after its discovery.

    Radial velocity discovery

    HD118203 b was found in 2006 by using the radial velocity technique: measuring the amount the star’s spectrum ‘wobbles’ tugged by its orbiting planet. Spectra are red shifted as the planet tugs its star away from us and blue shifted as the star is tugged towards us over one orbit of the planet. Radial velocity measurements give us the planet’s orbital period as well as its eccentricity and the minimum mass of the planet. The true planet mass depends on the relative inclination between star and planet. 43 radial velocity measurements from ELODIE* revealed HD118203 b as an eccentric planet with an orbital period of ~6.13 days, and a minimum mass of about 2 Jupiters (see Figure 1).

    2
    Figure 1: 43 radial velocity measurements by ELODIE showed the existence of HD118203 b (da Silva 2006). The top plot shows the radial velocity measurements over time. The bottom plot is phase folded with the period found by EXOFASTv2, and more clearly shows the periodic change in the star’s radial velocity by 100s m/s it is orbited by an object with minimum mass twice that of Jupiter. Figure 3 of today’s paper.

    While most orbit orientations will produce radial velocity signatures, only a small percentage happen to line up so that we can see the planet pass in front of its star, or transit. Transiting exoplanets block out a small fraction of light giving us the relative radii of planet and star. If a planet transits, this constrains the planet’s inclination and means that the minimum mass from radial velocity is very close to the true mass.

    A number of exoplanets discovered using radial velocities have since been found to transit but this process is time consuming for two reasons. First, only a small fraction of exoplanets will actually transit. Second, transits only last a very small fraction of the orbit (usually a couple of hours for an orbit of less than 10 days), so telescopes must stare at a star for long periods of time to discover when the transit actually occurs.

    TESS spies a transit (or five)

    NASA/MIT TESS replaced Kepler in search for exoplanets

    Transits of HD118203 b were discovered thanks to the ongoing TESS mission. TESS is a space mission which will observe most of the sky, staring at each sector for 28 days, looking for transiting exoplanets. Five transits were automatically identified using the Science Processing Operations Center (SPOC), see Figure 2, and following vetting to check for false positives, it was identified as a promising candidate.

    2
    Figure 2: TESS photometry of HD118203 b. The top plot shows the light curve as processed by SPOC and the bottom plot shows the flattened lightcurve as used in EXOFASTv2. Figure 1 of today’s paper.

    *ELODIE was an echelle type spectrograph installed at the Observatoire de Haute-Provence 1.93m reflector in south-eastern France for the Northern Extrasolar Planet Search.

    L’Observatoire de Haute-Provence, 1.93 meter telescope in the southeast of France. Altitude 650 m (2,130 ft)

    Its optical instrumentation was developed by André Baranne from the Marseille Observatory. The purpose of this instrument was extrasolar planet detection by the radial velocity method. This instrument was also used for the M-Dwarf Programmes.

    ELODIE first light was achieved in 1993. ELODIE was decommissioned in August 2006 and replaced in September 2006 by SOPHIE, a new instrument of the same type but with improved features.

    The SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes, literally meaning “spectrograph for the observation of the phenomena of the stellar interiors and of the exoplanets”) échelle spectrograph is a high-resolution echelle spectrograph installed on the 1.93m reflector telescope at the Haute-Provence Observatory located in south-eastern France. The purpose of this instrument is asteroseismology and extrasolar planet detection by the radial velocity method. It builds upon and replaces the older ELODIE spectrograph. This instrument was made available for use by the general astronomical community October 2006

    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 1:23 pm on December 26, 2019 Permalink | Reply
    Tags: "Making Blue Photons in Dwarf Galaxies", Astrobites, , , ,   

    From astrobites: “Making Blue Photons in Dwarf Galaxies” 

    Astrobites bloc

    From astrobites

    Dec 25, 2019
    Caitlin Doughty

    Title: The Ionizing Photon Production Efficiency Of Lensed Dwarf Galaxies At z∼2
    Authors: Najmeh Emami, et al.
    First author’s institution: University of California Riverside, Riverside, CA

    Status: Open access on arXiv

    How does star light escape from galaxies? It may seem like a simple question on its face, but it becomes more complicated the more you think about it. When you see a picture of a galaxy in optical wavelengths it seems to be very bright, but switching to a UV image you see less light. This decrease in the amount of light escaping for shorter wavelengths becomes worse as you progress into the far and extreme-UV (i.e. shorter wavelengths), where it becomes particularly prone to being absorbed by neutral hydrogen gas. Because of this tendency, when this short-wavelength radiation is created by stars, which are surrounded by dense clouds containing neutral hydrogen, much or even all of it gets absorbed and blocked from view.

    The absorption of this light creates problems for understanding the reionization of hydrogen in the intergalactic medium (IGM) that took place more than 12 billion years ago.

    Reionization era and first stars, Caltech

    In short, this process consisted of ionizing radiation coming from…somewhere…and separating the IGM hydrogen atoms into individual protons and electrons. But if our observations indicate that not much hydrogen-ionizing light can escape from galaxies, then how can reionization even happen? Although there are alternative possible sources of radiation, galaxies are the current best candidate for causing reionization, and the fact that photon escape and this process can’t be reconciled would seem to indicate astronomers are missing something important about this portion of the Universe’s history.

    This quandary motivates many experiments looking at how light escapes from galaxies, and today’s paper focuses on how efficiently galaxies, particularly dwarf galaxies, generate ionizing light and allow it to pass out into intergalactic space.

    Galaxy traits and UV photon production

    The goal of today’s featured paper is to determine the average efficiency of ionizing photon production from dwarf galaxies and to also evaluate their total photon output as an entire population. The average can be compared to that calculated for larger galaxies, which have already been studied for some time in this manner, and the total will quantify the contribution of small galaxies to reionization.

    The efficiency of ionizing light is the ratio of production of ionizing UV to the non-ionizing UV luminosity density. To calculate this, the authors collect observations of a sample of dwarf galaxies from about 11 billion years ago magnified by foreground galaxy clusters via an effect called gravitational lensing to measure their brightnesses at a few wavelengths: Hα, [O III], and 1500 Angstroms (a generic ultraviolet line).

    Large Magellanic Cloud. Adrian Pingstone December 2003

    smc

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

    Gravitational Lensing NASA/ESA

    3
    Figure 1: The efficiency of ionizing photon production (indicated with ζion on the y-axis) as a function of galaxy stellar mass (M* on the x-axis). There is no correlation with mass, but the young galaxies from this paper (red filled and empty circles) show generally higher values than those from a modern sample (green filled and empty circles). Purple circles from even earlier times also have higher efficiencies.

    No relation was found between the efficiencies of individual galaxies and their stellar masses (Figure 1 above), their total UV brightness, or even their brightnesses at short wavelengths relative to long (also called their UV spectral slope), and the dwarf galaxies were found to have similar efficiencies to more massive galaxies. However, there is a correlation with the efficiency and a galaxy’s brightness in both the Hα and [O III] emission lines, and the correlation is stronger for Hα (Figure 2 below). This suggests that galaxies that are brighter in these emission lines are also more efficient at generating ionizing radiation, and this may be related to the presence of very young stars, less than 100 million years old.

    4
    Figure 2: The efficiency as a function of [O III] emission (left panel) and H α emission (right panel). There are positive correlations of the efficiency with both lines, with a fit line shown in red and the 1 sigma uncertainty in light pink shading.

    When compared to similarly-sized galaxies from later in cosmic history (modern, nearby galaxies to be specific), they found that their sample seems to have a higher efficiency, indicating that there might be some evolution in time with how good dwarf galaxies are at making ionizing photons (Figure 1).

    The explanation for this might lie in the amount of iron contained in stars. A neutral iron atom has many electrons, which are good at preventing photons from making it out of a star’s atmosphere. However, iron is made mostly by Type Ia supernovae which take a long time to occur, and this means there is a time-delay for iron production in galaxies. Thus, galaxies from earlier in time have a deficit in iron, so their stars may be able to let out more ionizing photons compared to modern galaxies.

    The difference in efficiency with cosmic time could also be due to more bursts in the star formation rate or differences in typical stellar populations (for example, changing fractions of binary stars), but that cannot be determined without a more extensive analysis of these galaxies’ star formation behavior throughout cosmic time.

    If galaxies earlier in time were better at generating ionizing photons, then perhaps this could reconcile why both (1) observations of modern galaxies don’t show much evidence of ionizing photon escape but (2) reionization still managed to complete. In the future, the authors hope to further investigate the cause of the scatter in the efficiencies of dwarf galaxies to nail down the intricacies of photon escape.

    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 12:24 pm on December 25, 2019 Permalink | Reply
    Tags: "The “Shocking” Mystery about Filaments", Astrobites, , , , , , Shock,   

    From astrobites: “The “Shocking” Mystery about Filaments” 

    Astrobites bloc

    From astrobites

    Dec 24, 2019
    Michael Foley

    Title: The isothermal evolution of a shock-filament interaction
    https://arxiv.org/abs/1912.05242
    Authors: K. J. A. Goldsmith and J. M. Pittard
    First Author’s Institution: School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

    Status: Open access on arXiv

    Galaxies are built from black holes, stars, gas, and dust. While the stars and black holes may get most of the attention, the gas and dust play crucial roles in the evolution of a galaxy. Gas and dust are responsible for forming stars, feeding the central supermassive black hole, and regulating the chemical composition of the galaxy. Consequently, understanding the dynamics of the gas and dust is very important if we want to learn how galaxies work.

    One critical mechanism in the workings of galaxies is shocks.

    This X-ray image was produced by combining a dozen Chandra observations made of the central region of the Milky Way. The colors represent low (red), medium (green) and high (blue) energy X-rays. Chandra’s unique resolving power has allowed astronomers to identify thousands of point-like X-ray sources due to neutron stars, black holes, white dwarfs, foreground stars, and background galaxies. What remains is a diffuse X-ray glow extending from the upper left to the lower right, along the direction of the disk of the Galaxy. The Chandra data indicate that the diffuse glow is a mixture of 10-million-degree Celsius gas and 100-million-degree gas. Shock waves from supernova explosions are the most likely explanation for heating the 10-million degree gas, but how the 100-million-degree gas is heated is a mystery.

    Gas in galaxies can frequently become supersonic, such as when a supernova explodes, meaning that it travels faster than the local sound speed.

    Bullet Cluster NASA Chandra NASA ESA Hubble, evidence of shock

    This supersonic gas will generate shocks, just like an airplane traveling supersonically will produce a shockwave in the air that creates a sonic boom. The interactions of these shocks with other shocks or gas features can create turbulence or interesting substructures in the interstellar medium, potentially laying the fertile ground for stars to form.

    Today’s paper looks at the interaction of these shocks with filaments of gas. Filaments are long, coherent structures that are found throughout the interstellar medium and that serve as the birthplaces of stars (Figure 1). We know that filaments can be destroyed by shocks, but the exact conditions necessary to destroy a filament remain a mystery. The authors of ran a number of simulations to try to recreate these crime scenes.

    1
    Figure 1: A 50-light-year long filament of star-forming gas found in the Orion Nebula. Image: R. Friesen, Dunlap Institute; J. Pineda, MPE; GBO/AUI/NSF. Taken from the Dunlap Institute.

    Interstellar Medium
    While the space between the stars of the Milky Way Galaxy appears empty, it is actually filled with gas and dust which, together with stars, form a galactic ecosystem called the Interstellar Medium (ISM)—a dynamic landscape of vast, turbulent structures, radiation, nucleosynthesis, shockwaves, and stellar birth and death.

    The ISM is comprised mostly of gas, existing as ions, atoms or molecules. The gas is predominantly hydrogen, but there is also helium, both the products of primordial nucleosynthesis. The ISM also contains trace amounts of carbon, oxygen and nitrogen. Ionized gas is heated to millions of degrees K, while cold molecular, star-forming gas sits at temperatures measured in tens of degrees K.

    From this rich and dynamic medium come stars which form from dense concentrations of molecular clouds. Then, in a galactic circle of life, they replenish the ISM through the stellar wind they generate, and through supernovae and neutron star mergers that synthesize and disseminate heavier elements into the galaxy.

    The ISM is shaped by many forces: stellar radiation, stellar winds, as well as shockwaves from supernovae that can create “bubbles” in the ISM. Turbulence and magnetic fields also play roles in shaping this environment.

    As such, questions about the birth and death of stars are intricately linked to questions about the ISM. What’s more, a better understanding of the medium is critical in understanding extra-galactic phenomena, like the Cosmic Microwave Background, because we cannot observe them without peering through the filter of the ISM.

    At the Dunlap and U of T:

    Prof. Bryan Gaensler
    Dr. Cameron van Eck
    Dr. Jennifer West
    Jessica Campbell

    How did the authors gather clues? It’s filamentary, my dear Watson.

    To investigate this interaction, the authors conducted simulations of a single shock that crashes into a single filament. In order to better understand the physics, they varied multiple properties in the different simulation runs: the speed of the shock, the density of the filament, the orientation of the filament relative to the shock, and the length of the filament. By looking at how the remnants of the filament changed after varying certain parameters, they can gain clues about which parameters really matter for destroying a filament. Plots of one of these simulations are shown in Figure 2, where a filament is oriented sideways, or parallel to, the shock front. This filament is essentially blown up over time, and it develops interesting turbulence in its wake.

    2
    Figure 2: Filament (100 times more dense than surrounding material and oriented sideways to the shock) being hit by a shock (traveling at 3 times the speed of sound). The gas is color-coded by density, with red corresponding to high-density material and blue representing diffuse material. The development of turbulence and a ‘three-rolled’ structure in the filament material is clear in the last two snapshots. Taken from Figure 2 of the paper.

    The authors found that the filament gas behaved differently when they changed the orientation of the filament relative to the shock, the speed of the shock, and the length and density of the filament. For example, much less turbulence developed when the filament was closer to perpendicular to the shock (Fig 3). Additionally, the filament in Figure 3 was slowly stripped of its material rather than blown up somewhat rapidly like the filament in Figure 2. This means that we could potentially determine the orientation of a filament relative to a shock with only the properties of the remnant gas in the wake.

    3
    Figure 3: Plot of shock (traveling at 3 times the speed of sound) moving through a filament (100 times denser than the surrounding material and oriented at a 60 degree angle to the shock). The shock front is marked by the transition between light grey and white, and it is moving to the right. The filament is shown in dark grey, and each of the 7 panels depicts a single time snapshot. The authors model the shock as a continuous inflow of material, so the simulation remains full of shock material (light grey) even after the shock front reaches the other end. Taken from Figure 5 of the paper.

    Verdicts

    The authors notice many of these trends among the different properties that they tested. They found:

    Filaments oriented closer to perpendicular to the shock had longer and less turbulent wakes.
    Only sideways-oriented filaments developed a three-rolled structure (see Fig. 2), and filaments with densities different than that of the filament in Figure 2 formed more of a ‘C’ shape.
    Faster shocks stripped material from the filament much more quickly than slower shocks did.
    Longer filaments ended up moving faster than shorter filaments since they were exposed to more of the shock.
    These isothermal simulations were able to push the filaments much faster than in adiabatic simulations.

    These results are important for understanding the impact of shocks moving through the interstellar medium. Throughout their journey, they may encounter filaments with various properties. This work will help us to begin to understand what types of shocks are responsible for destroying various types of filaments, bringing us one step closer to solving this ‘shocking’ mystery.

    See the full article here .


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    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 December 10, 2019 Permalink | Reply
    Tags: "You Spin Me Right Round: Stellar Rotation with Asteroseismology", Astrobites, , , ,   

    From astrobites: “You Spin Me Right Round: Stellar Rotation with Asteroseismology” 

    Astrobites bloc

    From astrobites

    1
    Asteroseismology, or the study of sound waves bouncing inside of a star, can reveal information about the rotation of the star’s core. [Gabriel Perez Diaz, Instituto de Aastrofisica de Canarias (MultiMedia)]

    Title: Core-Envelope Coupling in Intermediate-Mass Core-Helium Burning Stars
    Authors: Jamie Tayar et al.
    First Author’s Institution: Institute for Astronomy, University of Hawaii

    Status: Accepted to ApJ

    All stars in nature rotate, including our own. However, stellar rotation over a star’s lifetime remains poorly understood. This has a profound impact on the accuracy of stellar models, which are our primary source for understanding the interiors and evolution of stars.

    Today’s paper focuses on internal rotation mechanisms; specifically, how a star’s core rotates with respect to its surface. Understanding stellar core rotation can teach us a ton about internal stellar physics and long-term angular momentum transport within a star’s interior.

    2
    Asteroseismology uses different oscillation modes of a star to probe its internal structure and properties. [Tosaka]

    A Problem of (Astero)Seismic Proportions

    Like many outstanding problems in astronomy, this problem can be solved by obtaining more data. How do we get more data on the internal core rotation rates of stars? Through asteroseismology! By studying stellar pulsations, we can infer information about a star’s interior.

    The authors of today’s paper focused on evolved intermediate-mass stars, or stars between two and eight times the mass of the Sun. These stars fall in the transition region between low- and high-mass stars, as their name implies. Like their more massive counterparts, these stars have a convective core and rotate rapidly during the main sequence — the phase of evolution where stars burn hydrogen into helium. However, like low-mass stars, intermediate-mass stars become cool red giants as they evolve. It turns out red giant stars also pulsate like the Sun, a low-mass star. By comparing how red giant stars oscillate to how the Sun oscillates, we can measure stellar parameters for red giants, such as their mass and radius.

    The Core Tells All

    We can additionally infer core rotation periods for red giant stars using asteroseismology, making them the perfect candidates for this study. In red giant stars, waves that propagate near the stellar core interfere with waves that propagate on the surface. By measuring surface pulsations, we can determine how the core and surface waves interact. From there, we can infer details about the stellar core, such as rotation.

    3
    Figure 1: Stellar cores spin more slowly as intermediate-mass stars evolve, as shown by this comparison between core rotation period and surface gravity. [Tayar et al. 2019]

    After measuring the core rotation periods for the stars in this sample via asteroseismology using data from the Kepler Space Telescope, the authors compared their rotation periods with several other stellar parameters and analyzed how stars with these measured core rotation periods should evolve over time. Figure 1 shows a correlation between measured core rotation periods and surface gravity, which decreases as stars of the same mass evolve. This trend with surface gravity indicates that as these stars evolve, their cores rotate more slowly. The authors also compared their measured core rotation periods with stellar mass and metallicity but found no obvious trends.

    Several of the stars in the sample also had surface rotation periods measured by a previous study. This comparison is shown in the left panel of Figure 2. This comparison suggests that as stars decrease in surface gravity (evolve), the ratio between their core rotation period and measured surface rotation period gets closer to 1 (i.e. the surface and core rotation periods become more similar as a star evolves), indicating that the stellar core can become recoupled with the surface as time goes on. The authors, however, exercise caution with such a result. When they predict surface rotation periods with stellar models, that obvious trend disappears (right panel of Figure 2) which shows that there may be a bias when selecting stars with measured surface rotation periods.

    4
    Figure 2: Surface rotation periods measured from starspot modulation show a trend when compared to core rotation periods and surface gravity (left) while surface rotation determined by models does not (right). [Tayar et al. 2019]

    Evolving Stellar Astronomy

    The results of this study have several implications for our understanding of stellar evolution. The evolution of core rotation periods over time suggests angular momentum transport occurs between the core of the star and the surrounding envelope. The comparison with surface rotation periods also shows some evidence for core-surface recoupling as these stars evolve. This study provides insight into internal stellar rotation that can be used to improve current stellar models and provides a new jumping off point for future work.

    See the full article here .


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    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 3:04 pm on December 9, 2019 Permalink | Reply
    Tags: "Uncovering New Sources of Dwarf Galaxy Feedback", Astrobites, , , ,   

    From astrobites: “Uncovering New Sources of Dwarf Galaxy Feedback” 

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    From astrobites

    Dec 9, 2019
    Keir Birchall

    Title: AGN-Driven Outflows in Dwarf Galaxies
    Authors: Christina Manzano-King, Gabriela Canalizo, and Laura Sales
    First Author’s Institution: Department of Physics and Astronomy University of California Riverside, USA

    Status: Accepted in The Astrophysical Journal, open access on arXiv

    Feedback doesn’t just appear on your homework or as the horrible screeching noise in poorly assembled speakers, it also helps shape the evolution of a galaxy. Feedback consists of wind or jet-driven motion of gas and dust within or without a galaxy from many different sources: supernova explosions, radiation from stars or outflows from active galactic nuclei (AGN). Winds driven by these sources blow throughout the galaxy enriching its gas with metals and regulating its star formation. Stellar sources were, until recently, believed to dominate feedback processes in dwarf galaxies, largely because it was thought such low mass galaxies couldn’t host AGN. Over the past decade, however, there have been hundreds of detections of AGN within dwarf galaxies. Motivated by this plethora of detections, today’s authors set out to identify the dominant source of feedback in nearby dwarf galaxies and report some of the first direct detections of AGN-driven outflows.

    Constructing the Sample

    As a starting point, today’s authors make use of this wealth of detections by focusing on papers that identified AGN in the infrared and optical parts of the spectrum. Outflows from either star formation or an AGN will ionise the surrounding gas and leave distinct patterns in the host galaxy’s spectrum. As a first step in constructing their sample, the authors employed the BPT diagnostic (named after its creators Baldwin, Phillips & Terlevich). It compares the ratio of two optical emission line pairs to determine whether the host galaxy’s spectrum is dominated by AGN processes, star-formation processes or is a composite of both. Using this today’s authors constructed two galaxy samples: those dominated by AGN emission and those dominated by star formation processes. Within each of these samples, the authors prioritised spatially extended galaxies which allowed them to measure the galaxy’s spectrum at various positions along its axis. An illustration of this is shown in figure 1. With these criteria, the authors construct a sample of 29 AGN and 21 star formation dominated galaxies and used the Keck telescope to measure spectra at discrete positions along its axis. Additionally, they took an average spectrum across the full extent of the galaxy.

    Keck Observatory, operated by Caltech and the University of California, Maunakea Hawaii USA, 4,207 m (13,802 ft)

    2
    Figure 1: Illustration of the direction of spectral measurement on a selection of the galaxy sample used in today’s work. The white lines represent the orientation of the spectral slit. Adapted from figure 2 in today’s paper.

    Finding the Flow of an Outflow

    Emission lines in these spectra were fit using two approaches shown in the top row of figure 2. In the left-hand panel, each peak in the doublet is fit using a single Gaussian whereas the right-hand panel uses a broad (orange distribution) and narrow component (green distribution) to fit each peak. The χ2 value, in the top-left hand corner of each panel, measures the difference between the observed and fit distributions, so the much lower value in the right-hand panel tells us that using two distributions produces the most accurate fit. By fitting two distributions to a peak, the authors can track both the gas bound in the bulk of the galaxy using the narrow component and capture any potential outflows with the broad component. Outflows are generally described as gas moving out of the galaxy, thus at different velocities relative to bound gas in the galaxy. Motion from outflows would appear in the spectrum as Doppler broadening, so any detectable broadening or shift in these components relative to one another would be indicative of a potential outflow, as can be seen in the bottom panel of figure 2.

    3
    Figure 2: Illustrating the fitting process to the [OIII] doublet. The top panel shows that using two Gaussian distributions to fit each line produces a better fit to the doublet, shown by the reduced χ2 value. The bottom panel highlights the different characteristics of the narrow (green) and broad (orange) distributions. As discussed in the text, fitting two distributions allows the authors to measure the velocity and intensity of the outflow using the broad, orange distribution. The broadening and offset from the narrow distribution suggests we are seeing outflow activity. Adapted from figure 3 in today’s paper.

    Of the 50 galaxies in their sample, 15 had detectable broad components indicative of gas moving independently of the galaxy. For these 15 galaxies, the spectral fitting process was repeated using the spectra measured at various points along the extent of each galaxy. 13 of these 15 galaxies had broad components at all radii and thus considered to have outflows. Using the best fit model from the averaged spectra, the BPT classification was determined for both the narrow and broad components, and plotted in figure 3. From this the authors determine the primary source of ionisation for the bound gas (smaller, fainter symbol) and the source of the outflow (larger symbol). The authors find 9 galaxies are dominated by AGN emission, of which 7 have AGN-driven outflows. These are among the first direct detections of AGN-driven outflows in dwarf galaxies.

    4
    Figure 3: BPT diagnostic characterising the primary source of ionisation in gas bound in the galaxy (small, fainter points) and that driving the outflows (larger, brighter points). The points that fall into the top-right region are galaxies and outflows dominated by AGN, the bottom-left region has galaxies and outflows dominated by star formation and those in-between are a composite of both. This is figure 5 in today’s paper.

    The Nature of the Flow

    Averaging the broad components for star formation and AGN-driven outflows highlights some crucial differences. Figure 4 shows that AGN outflows are, on average, blueshifted compared to the narrow component. Since AGN generally lie in the denser centre of a galaxy, any material moving away from the observer, and hence redshifted, would be obscured, leaving a net blueshift. AGN-driven outflows also carry a greater proportion of the flux relative to star formation driven outflows. This suggests that AGN-driven winds could remove larger amounts of gas from the galaxy than those driven by star formation. Thus, AGN are likely to play a significant feedback role in dwarf galaxies.

    5
    Figure 4: Results of the fitting for the BPT-classified AGN-driven (above) and star formation driven outflows (below). Averaging the broad components shows clear differences between the nature of these flows. Greyed out panels were not included in the ‘AGN average’ panel as these are the galaxies believed to host AGN but shows signs of star formation driven outflows. Adapted from figure 6 in today’s paper.

    Today’s authors have utilised the plethora of new AGN found in dwarf galaxies to make the first direct detections and measurements of AGN-driven outflows in these low mass hosts. Despite their small sample, the authors have established a set of vital constraints that will help inform future galaxy feedback simulations. Alongside an ever-increasing number of AGN detections, this work highlights the significance of AGN in the evolution of dwarf galaxies.

    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 3:21 pm on December 4, 2019 Permalink | Reply
    Tags: "Searching for Supernova Survivors", Astrobites, , , , , There are two kinds of Type Ia SNe both caused by white dwarfs hitting the Chandrasekhar mass limit — single degenerate (SD) and double degenerate (DD).   

    From astrobites: “Searching for Supernova Survivors” 

    Astrobites bloc

    From astrobites

    12.4.19 from AAS NOVA
    Lauren Sgro

    1
    Artist’s impression of a supernova. Could some companions survive these powerful explosions? [ESO/M. Kornmesser]

    Title: Search for Surviving Companions of Progenitors of Young LMC Type Ia Supernovae Remnants
    Authors: Chuan–Jui Li et al.
    First Author’s Institution: National Taiwan University
    3
    Status: Accepted to ApJ

    Supernovae Survivors?

    Surviving a supernova (SN) may sound crazy, since supernovae (SNe) are among the most energetic events in space. Type Ia SNe result from the explosions of white dwarfs, and just one of these events can temporarily outshine an entire galaxy. So how could anything survive such an explosion?

    Well, there are two kinds of Type Ia SNe, both caused by white dwarfs hitting the Chandrasekhar mass limit — single degenerate (SD) and double degenerate (DD). DD Type Ia Sne are caused by the merger of two white dwarfs that, upon merging, will pretty much annihilate one another and cause a SN. However, a SD Type Ia SN only involves one white dwarf. In this case, there is no merger; instead, the white dwarf has a non-degenerate (a.k.a., not a white dwarf) companion from which it has drawn too much mass, causing the white dwarf to explode. Since only one star (called the “progenitor”) is doing the exploding in this SD scenario, perhaps that companion will live long enough to tell its story…

    3
    Figure 1: SN 0519–69.0. The authors fit the SN shell in Hα (taken by HST) to an ellipse marked in white, with the white cross at its center. Averaging this with the center determined by another publication, the authors take the red ‘x’ as the explosion center. The red dashed circle marks the runaway distance for a MS companion (0.2 pc) and the cyan circle marks this distance for helium companions (0.6 pcs). The green circle denotes the search radius for background stars taken between the cyan and green circles. Similar figures for the other supernovae are available in the paper. [Li et al. 2019]

    Searching for a Companion

    The authors of today’s paper set out to look for potential companions dancing around SN remnants, the shells of material left over by SN explosions. The sought-after companions, which could be main sequence (MS) stars, red-giant stars, or helium stars, may have lost their outer layers in the deadly explosion but could live on as a dense core. These surviving cores should be identifiable — they probably move differently as a result of the explosion, and they likely look different in color.

    Knowing that these companion cores will stand out from background stars, the authors choose three Type Ia supernovae remnants to investigate for survivors: SN 0519–69.0, DEML71, and SN 0548–70.4. Because SN remnants in our own galaxy can be tough to look at through the galactic plane, these remnants are all located in the Large Magellanic Cloud (LMC).

    Large Magellanic Cloud. Adrian Pingstone December 2003

    The first two SNe on the list have been examined before with no luck, but the authors hope that their new Hubble Space Telescope data will shed new light on these areas of the sky.

    Today’s authors use those two methods, analyzing the color and the motion of stars surrounding the chosen SNe to search for surviving companions. Before they can do this though, they need to determine a proper area to search.

    Where to Look?

    SNe remnants have a generally circular or elliptical shape, as the shock from the explosion propagates outward in all directions and interacts with the interstellar medium. By finding the geometrical center of the remnant’s visible shell, the authors estimate an explosion site (see Figure 1).

    If a star survives a SN explosion, its velocity after the supernova should be the sum of its own orbital velocity and the velocity of the progenitor’s translational velocity. Previous studies have determined the maximum speed that a MS or helium star could be traveling after a Type Ia SN. Using these velocities, the authors calculate just how far a companion core could have traveled away from the SN center since the explosion and narrow their search for survivors to this area (called the “runaway distance”). And of course, there has to be a control — the authors determine a set of background stars to which they can compare their potential survivors (see Figures 1 & 2).

    4
    Figure 2: SN 0519–69.0. Stars with V mag < 23.0 (most likely cutoff for potential companions from Schaefer et al. 2012) that lie within the runaway bounds. These are analyzed as potential survivors in the CMDs and RV plots. Red for MS, cyan for helium stars. [Li et al. 2019]

    Method 1: Examining Color

    To examine the color of their potential survivors, the authors plot the stars’ colors and absolute magnitudes on a very useful diagram called a color–magnitude diagram (clever name, right?). Included on these plots are all the candidate companions and background stars, as well as several “post-impact evolutionary tracks” (see Figure 3). These tracks are merely paths on the diagram that show how a MS or helium companion star, after a SN explosion, should change in color (which depends on its temperature) and brightness according to its initial mass. Therefore, if there are any true surviving companions, they should lie on these tracks.

    You may have noticed that red-giant stars, although a potential type of companion, have not been included in the search up to this point. Astronomers do not yet have evolutionary tracks for red giants, unfortunately. More on why that is unfortunate in just a second.

    5
    Figure 3: CMDs for SN 0519–69.0. Left: The HST equivalent of a V vs B–V CMD. Right: The HST equivalent of an I vs V-I CMD. Evolutionary tracks are shown in green, with the helium star tracks situated in the left of each diagram. [Li et al. 2019]

    Method 2: Examining Motion

    The second method for identifying surviving companions is to examine their radial velocity (RV), the speed of their motion away from or towards the Earth. Astronomers need spectral data to get this, which the authors only have for SN 0519–69.0 and DEML71. Now, although we don’t have a great idea of what that RV should be, it clearly should be different from the RV of background stars not involved in the SNe. The authors look at the distributions of RVs for relevant stars (candidates or candidates+background — Figure 4) to determine which stars have abnormal RVs, and these are considered candidate survivors.

    6
    Figure 4: RV for stars with V mag < 21.6 (limiting magnitude for reliable spectral fits). For SN 0519–69.0, there were only a few candidates, so the authors included the background stars to establish a distribution. Star #5 is the strange one — it is not moving with the rest of the group! Again, the same figures for the other SNe are available in the paper. [Li et al. 2019]

    Results

    So what came of this survivor search? Let’s take a look at each supernova.

    SN 0519-69.0: The CMD search did not return any potential companions. The stars within the runaway radii have colors that do not fall on one of the corresponding evolutionary tracks. However, there is a star with a strange (> 2.5σ away from the mean) RV, as shown in Figure 4. This oddball star may be considered a candidate if it also fell on the evolutionary tracks, but it does not. Why, you ask? Well, it seems that this star is likely a red giant, as it falls on the red giant branch in the CMDs. So, this star could very well be a candidate, but red-giant evolutionary tracks must be developed for the authors to confirm either way (that’s the unfortunate part).

    DEML71: This SN has a very similar story to SN 0519-69.0. No stars can be considered candidates from the CMDs, but there is indeed a star with a strange RV. However, as we saw before, it seems to be a red giant and therefore cannot be considered a candidate due to the lack of theoretical data. Boo.

    SN 0548-70.4: Inspection of the CMDs show that there is indeed a star that falls on one of the MS evolutionary tracks! Great! … But wait… there’s more. This star does not appear on evolutionary tracks for both colors, so the authors remain skeptical — a true candidate should fall on tracks for both CMDs. Furthermore, the part of the evolutionary track that the candidate does fall on indicates an age of only ~110 years. This SN remnant is about 10,000 years old, so obviously this star is unrelated to the explosion and is likely not the candidate the authors were looking for.

    As with all science, null results are still results. Even though no surviving cores were identified, the authors still gained valuable information — like, we really need some red-giant post-impact evolutionary tracks. Or perhaps these SNe are not what they seem; if the SD and DD models are drastic oversimplifications, then our predictions for them won’t lead us to surviving stars. Many other types of Type 1a supernova have been proposed, such as sub-/super-Chandrasekhar or spin-up/spin-down. All in all, astronomers rely on models quite often, since we can’t go grab a star. With comparison to more models, we will have a better picture of reality.

    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 1:25 pm on December 1, 2019 Permalink | Reply
    Tags: "A Dizzying Diversity of Dwarf Galaxies", Astrobites, , , ,   

    From astrobites: “A Dizzying Diversity of Dwarf Galaxies” 

    Astrobites bloc

    From astrobites

    Nov 29, 2019
    Tomer Yavetz

    Title: Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves
    Authors: Isabel M.E. Santos-Santos, Julio F. Navarro, et al.
    First Author’s Institution: Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada
    1
    Status: Submitted to MNRAS, open access on arXiv

    The known saying goes, “where there’s smoke, there’s fire.” Just like we infer the existence of a fire when we see smoke, we infer the existence of mass when we see objects moving in circles. Physics dictates that objects will revolve around a large mass — the larger the mass, the faster the rotation. When the observable matter (stars, dust, gas, etc. — collectively referred to as baryonic matter) is not sufficient to explain the rotational velocities, most astronomers conclude that there must be some form of invisible matter, or dark matter.

    A successful theory needs to account for the rotational velocities as a function of radius (aka the galaxy’s “rotation curve”), which requires a precise knowledge of the density profile of baryons and dark matter. For most galaxies, ranging in size from galaxies like our own Milky Way to huge galaxy clusters with hundreds of Milky Way-sized galaxies, a simple model for dark matter is sufficient, and it is enough to know one parameter (like the total mass of the system, or the maximum rotational velocity), in order to accurately predict the full shape of a galaxy’s rotation curve.

    However, the situation gets much more complicated with dwarf galaxies. Figure 1 shows the rotation curves of four dwarf galaxies that should, in theory, look similar. In some cases, the rotational velocities climb much faster than expected, meaning the central density is higher than our theories predict. In other cases, the rotational velocities climb surprisingly slowly, meaning the center of these galaxies must be less dense than we might expect.

    2
    Figure 1. Observed rotation curves of four dwarf galaxies. The grey region surrounding the black line indicates the expected rotation curve from Cold Dark Matter (based on an NFW profile). The red dots represent the observed rotational velocities at each radius, while the grey dots mark the fraction of the rotational velocity that is caused by the baryonic mass component. In the top left panel, the theoretical curve severely underpredicts the density in the center of the galaxy, while on the bottom right the density is heavily overpredicted. The parameter ηrot signifies the ratio between the rotation velocity near the center of the galaxy (the fiducial radius, marked by the vertical dotted line) with the rotation velocity at its outer edge (Figure 1 in today’s paper).

    1.Baryonic physics: Baryonic effects like feedback can create strong outflows of baryons from the inner regions of a galaxy, leading to a reduction in the dark matter content in those regions (causing rotation curves like the bottom right panel in Figure 1). At the same time, inflows of cold gas can have the opposite effect, increasing the density at the center of the galaxy and deepening the potential well (causing a rotation curve like the top left panel in Figure 1).
    2.Dark Matter physics: several theories predict that dark matter may be influenced by forces other than gravity, leading to deviations from the predicted rotation curves in Figure 1. One such example is Self-Interacting Dark Matter (SIDM), which is expected to form shallower potential wells (leading to rotation curves similar to the bottom right panel in Figure 1).
    3.Baryonic acceleration laws: some theorists have argued that the rotation curves of galaxies can be explained using only the spatial distribution of the baryonic components of galaxies, and that the contribution of the dark matter to the rotation curve is fully specified by that of the baryons. Such acceleration laws can (but don’t necessarily have to) arise from theories of modified gravity that do away with dark matter altogether.
    4.Observational uncertainties: uncertainties in the circular velocities used to measure rotation curves can arise if a large fraction of the orbits in the central region of the galaxy are non-circular. This is especially likely if the shape of the potential is triaxial instead of spherical. Such uncertainties can cause both under- and over-estimation of the central densities of dwarf galaxies, leading to the diversity of rotation curves shown in Figure 1.

    The main goal of today’s paper is to evaluate each of these theories against the existing observational data. The authors make use of simulations (APOSTLE and NIHAO) to test whether any of the theories are able to reproduce the observed diversity of rotation curves. In each case, they compare the expectations from theory/simulations with the observed data. Figure 2 shows one such example to test the theory of a baryonic acceleration law (category #3 above).

    4
    Figure 2. Comparison of observed data to the predicted rotation curves based on a baryonic acceleration law. The x-axis is the acceleration calculated from the baryonic components at the fiducial radius, while the y-axis is the observed acceleration at the same radius. The blue region indicates the expectation based on a baryonic acceleration law. Many of the observed galaxies, especially those with low ηrot, deviate significantly from the prediction (Figure 6 in today’s paper).

    So which of the four theories can justify the observed rotation curves? The authors argue that the answer is ‘none of the above.’ While each theory was able to explain certain galaxies, none of the theories reproduced the full diversity of the observed rotation curves without requiring additional assumptions (as shown in Figure 2 for the baryonic acceleration law explanation) .

    It is very common for papers (and Astrobites) to end with some variation of the statement, “we need more data!” Today’s paper ends on a very different note — the data we already have highlights a clear issue with our current understanding of how dwarf galaxies behave. What we need is not more data, but rather a more satisfying theory to explain the dizzying diversity of dwarf galaxy rotation curves.

    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 3:28 pm on November 26, 2019 Permalink | Reply
    Tags: "Not Far in the Dark", Astrobites, , , , , Dunlap Institute Dragonfly telescope team, Galaxy NGC 1052–DF2 had almost no dark matter. The existence of an object like NGC 1052–DF2 has enormous implications for models of galaxy formation and behavior., Tip of the Red Giant Branch (TRGB) stars   

    From astrobites: “Not Far in the Dark” 

    Astrobites bloc

    From astrobites

    26 November 2019
    Tarini Konchady

    Title: A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy NGC 1052–DF4 from Deep Hubble Space Telescope Data
    Authors: Shany Danieli et al.
    First Author’s Institution: Yale University

    Status: Submitted to ApJL

    1
    NGC 1052–DF4 as seen by the Hubble Space Telescope. [Danieli et al. 2019]

    At the end of March last year, the Dragonfly team announced the discovery that galaxy NGC 1052–DF2 had almost no dark matter (see this Astrobite for more on the discovery).

    U Toronto Dunlap Dragonfly telescope Array at its home at high-altitude observing location New Mexico Skies hosting facility at 7300′ altitude

    This announcement set off a flurry of responses, since the existence of an object like NGC 1052–DF2 has enormous implications for models of galaxy formation and behavior (see this Astrobite for a great summary of some of the initial responses).

    3
    Figure 1: A color–magnitude diagram of globular cluster Messier 55 (M55). The TRGB can be seen at the upper right. [B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope]

    Carnegie Institution 1-meter Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile

    To determine how much dark matter a galaxy has, you have to compare its stellar mass (which comes from estimating how many stars it has) and its dynamical mass (which comes from measuring how the contents of a galaxy are moving). Measurements of stellar mass and dynamical mass are extremely dependent on distance, which was the basis of some criticisms of the NGC 1052–DF2 discovery paper.

    Tip of the Red Giant Branch (TRGB) stars (see Figure 1) are stars that have just run out of hydrogen and started to burn helium. Being at this turning point in their lives gives TRGB stars a characteristic brightness and color, meaning that they can be used to measure distances (like here). In today’s paper, members of the Dragonfly team use TRGB stars to measure the distance to NGC 1052–DF4, another seemingly dark-matter-deficient galaxy.

    Tipped Off (by) the Edge

    The authors base their study on observations taken by the Hubble Space Telescope (see the cover image above). To identify the TRGB stars, the authors first separate out RGB stars from the rest. They then group these RGB stars based on their brightness to get a luminosity function for the galaxy. The first derivative of the luminosity function is then used to identify the location of the TRGB stars (see Figure 2). This technique is called edge detection.

    The identified location of the TRGB doesn’t seem to shift too much with radius, and pegs the apparent magnitude of TRGB stars (in the I814 band) at 27.25 ± 0.11 mag. The absolute magnitude of TRGB stars in the I814 band is about -4.0 mag. Taken with the apparent magnitude and median color of the identified TRGB stars, this gives a distance of 18.3 ± 1.0 megaparsecs (1 megaparsec is ~3.26 million light years) to NGC 1052–DF4.

    4
    Figure 2. Color–magnitude diagram (CMD) of RGB stars in NGC 1052–DF4 (left), the luminosity function of the galaxy (middle), and the output of the edge detection algorithm (right). The gray points and black points in the CMD are the stars that were rejected and accepted respectively, following quality cuts. [Danieli et al. 2019]

    Not So Distant

    To understand and account for photometric errors (which edge detection is unable to do), the authors inject artificial stars into their observations and recover them. The authors then re-estimate the galaxy distance by modeling the galaxy using parameters obtained from their observations. They get a TRGB apparent magnitude of 27.31 ± 0.03 ± 0.09 (the first error spans the central 68% of the magnitude likelihood and the second error comes from systematic uncertainty) and a distance of 18.8 ± 0.9 megaparsecs.

    The distance to NGC 1052–DF4 the authors obtain agrees with the distance obtained via surface brightness fluctuations (how the brightness of a galaxy varies if you divided it into segments and compared them). However, their TRGB distance differs by varying degrees from TRGB distances obtained by other studies. The authors suggest that this might be due to non-TRGB stars being mistaken for TRGB stars.

    The TRGB distance obtained in this work places NGC 1052–DF4 in close proximity to NGC 1052 and NGC 1052–DF2, consistent with previous measurements. It also supports the interpretation that NGC 1052–DF4 is a dark-matter-deficient galaxy. NGC 1052–DF2 will receive similar treatment very soon, continuing the fascinating story of these strange galaxies.

    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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