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  • 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, , , ​Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope, , , ,   

    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.

     
  • richardmitnick 2:58 pm on February 22, 2020 Permalink | Reply
    Tags: "Through the Lens: Milky Matter Magnifies Magellanic Motion", Astrobites, , , , , , ,   

    From astrobites: “Through the Lens: Milky Matter Magnifies Magellanic Motion” 

    Astrobites bloc

    From astrobites

    Feb 22, 2020
    Luna Zagorac

    Title: First Results on Dark Matter Substructure from Astrometric Weak Lensing
    Authors: Cristina Mondino, Anna-Maria Taki, Ken Van Tilburg, and Neal Weiner
    First Author’s Institution: Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003, USA

    Status: pre-published on arXiv

    There is about five times more invisible Dark Matter than its luminous counterpart in the universe—but how do we go about detecting something that can’t be directly imaged?

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    The LSST, or Large Synoptic Survey Telescope is to be named the Vera C. Rubin Observatory by an act of the U.S. Congress.

    LSST telescope, The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

    Dark Matter Research

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Scientists studying the cosmic microwave background [CMB]hope to learn about more than just how the universe grew—it could also offer insight into dark matter, dark energy and the mass of the neutrino.

    [caption id="attachment_73741" align="alignnone" width="632"] CMB per ESA/Planck

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    Dark Matter Particle Explorer China

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

    LBNL LZ Dark Matter project at SURF, Lead, SD, USA


    Inside the ADMX experiment hall at the University of Washington Credit Mark Stone U. of Washington. Axion Dark Matter Experiment

    One way is to look for the gravitational effects of dark matter clumps on images of normal matter along the same line of sight. This type of effect is called gravitational lensing.

    Gravitational Lensing NASA/ESA

    In today’s paper, the authors specifically look for the effects of weak lensing from low-mass structures consisting entirely of dark matter.

    Weak gravitational lensing NASA/ESA Hubble

    The foreground dark matter structure creates a lens that bends the light coming towards an observer from some background luminous source. Unlike strong lensing, weak lensing doesn’t impact a single background source, but instead serves to preferentially align several background sources along some field. For more information on different types of lensing and how they work, check out this bite.

    Why Use Weak Lensing?

    Alignments of foreground and background sources that lead to weak lensing are much more common than those leading to strong lensing. Because low-mass dark matter structures are predicted to exist in the Milky Way, they should be both common in observational data sets and detectable through microlensing signatures. Furthermore, because such structures are completely devoid of normal matter, they pose a “pristine testing ground” for probing the microphysics of dark matter without the interference of normal, luminous matter.

    How to Look For Weak Lensing?

    1
    Figure 1: Diagram of gravitational lensing of sources i by lens l. Note the blue monopole pattern of the angular displacement \Delta \theta_{il}. This is not constant in time, leading to the red dipole pattern lensing corrections \Delta \mu_{il} to the sources’ proper motions \mu_i. This dipole pattern is universal, and is what the authors look for. Figure 1 in the paper.

    The authors use a template approach, which is similar to the one used when detecting astrophysical signals with LIGO. Figure 1 shows the dipole pattern of velocity corrections of background stars which stems from weak lensing. The exact shape and size of the template depend on the angular position \mathbf{\theta}_t, angular scale \beta_t, and effective lens velocity direction \hat{\mathbf{v}}_{t} of the dark matter lens. The details of the matched filter to the lens-induced velocity vector profile also include information about the density profile of the dark matter lens. This means that finding the correct shape of velocity corrections in the data and comparing its magnitude with the theoretical template model can inform the size, position, and density profile (and subsequently, mass) of the dark matter lens.

    Where to Look For Weak Lensing?

    The researchers looked to the Milky Way to provide the dark matter lenses, and extra-galactically to the Large and Small Magellanic Clouds (LMC, SMC) to provide the luminous matter to be lensed.

    Milky Way NASA/JPL-Caltech /ESO R. Hurt. The bar is visible in this image

    Large Magellanic Cloud. Adrian Pingstone December 2003

    smc

    Small Magellanic Cloud. NASA/ESA Hubble and ESO/Digitized Sky Survey 2

    They used the second data release from Gaia and chose the LMC and SMC data for their large stellar number densities and low proper motion dispersions, both intrinsic and instrumental.

    ESA/GAIA satellite

    This left the authors with a high signal-to-noise ratio, thus best equipping them to look for signatures of weak lensing.

    In order to look for the tell-tale dipole template motion, the authors cleaned the data up a bit. First, they subtracted overdense stellar clusters, as they generally move coherently and independently from the bulk stars in the Magellanic Clouds. Additionally, they subtracted the large-scale proper motion of the clouds themselves. Finally, they removed stars which are in the line of sight, but not bound to the clouds.

    3
    Figure 2: Average stellar proper motion per 0.03° pixels in the RA (left) and DEC (right) across the Large Magellanic Cloud. The top panel shows the proper motion in the original Gaia data sample after the removal of dense clusters; the bottom shows it after further background motion subtraction and removal of outlier stars. Figure 7 in the paper.

    What did the authors find?

    In performing their analysis, the authors produced exclusions on the fraction of dark matter present in lensing sources as a function of lens mass (see Figure 3). They also noted that the current analysis is statistics-limited, with their figure of merit being largest for relatively faint stars, such as the majority of those present in the Magellanic Clouds. Thus, the statistics in their analysis will improve with additional integration time, which is currently at 22 months for Gaia DR2. Furthermore, having a larger sample of stars, better resolution of binaries, and accurate modeling of telescope systematics will all lead to improvements over time, yielding promising prospects for the use of their method on future data releases from Gaia and other astrometric surveys.

    4
    Figure 3: Constraints from the Magellanic Cloud velocity template analysis on the fractional dark matter abundance f_l of compact objects with mass M_l with a given density profile. The three linewidths represent compact object radii r_{l}=10^{-3}, 0.5, \text { and } 1 \mathrm{pc}. The constraint for the smallest radius is equivalent to the one for point-like objects. Above the diagonal line at the bottom right, at least one subhalo eclipses the data sample with 90% confidence level (CL). Figure 5 in the paper.

    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 3:05 pm on February 18, 2020 Permalink | Reply
    Tags: "The TESS Mission’s First Earth-Like Planet Found in an Interesting Trio", Astrobites, , , ,   

    From astrobites: “The TESS Mission’s First Earth-Like Planet Found in an Interesting Trio” 

    Astrobites bloc

    From astrobites

    18 February 2020
    Haley Wahl

    1
    Artist’s illustration of what the exoplanet TOI 700 d might look like. [NASA’s Goddard Space Flight Center/Chris Smith (USRA)]

    Title: The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System
    Authors: Emily A. Gilbert, Thomas Barclay, Joshua E. Schlieder, et al.
    First Author’s Institution: University of Chicago

    Status: Submitted to AJ

    2
    Artist’s impression of TESS observing planets orbiting a dwarf star. [NASA Goddard SFC]

    Since the discovery of the first planet outside of our solar system in 1992, the field of exoplanets has been booming with interesting finds. From the diamond planet orbiting a neutron star to the giant pink planet orbiting a star in the constellation of Virgo, telescopes all over the world have been racing to find the latest gem. Of particular interest are Earth-like planets. A team led by a graduate student at the University of Chicago report the first Earth-sized planet in the habitable zone found by the TESS mission, and its surroundings were quite a surprise to astronomers.

    NASA/MIT TESS replaced Kepler in search for exoplanets

    Searching for Planets in All the Right Places

    Some of the biggest questions we humans like to ask are, “Is there life out there in the universe?” and “Are there other solar systems out there with planets just like ours?” To answer these questions, astronomers have built larger and more advanced telescopes to try to find planets outside of our own neighborhood, specifically those similar to our own world. The Kepler mission was launched in 2009 specifically to search for these kinds of planets: Earth-sized planets in Earth-like orbits around Sun-like stars in order to study how common they are in the universe.

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

    The mission has made many amazing discoveries, such as an exoplanet with the density of Earth, a planet in a binary star system, and the first Earth-sized planet in the habitable zone of its star that orbits around an M-dwarf star that is about half the size of the Sun. Kepler’s extended mission, K2, focuses on low-mass stars and has led to the discovery of hundreds of small planets, some even in the habitable zone of their stars. Together, Kepler and the K2 mission have found over 3,000 new exoplanets.

    Exoplanets are very small and very far away, so it is very difficult to find them. Astronomers use four methods: the transit method (which looks at how much a star dims as a planet goes in front of it, or eclipses it), the wobble method (which looks at how a planet and a star move around a common center of mass), direct imaging (which means taking a picture of the planet, straightforward but very difficult and limiting), and microlensing (which happens when light from a distant star bends around a star/planet system).

    Planet transit. NASA/Ames

    Radial Velocity Method-Las Cumbres Observatory

    Direct imaging-This false-color composite image traces the motion of the planet Fomalhaut b, a world captured by direct imaging. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute

    Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

    TESS, or the Transiting Exoplanet Survey Satellite, was launched in 2018 and was designed to search for small planets around the Sun’s nearest neighbors using the transit method. In today’s paper, we discuss the first results of Earth-sized planets found in the habitable zone of an M-dwarf star, planets contained in an odd planetary trio.

    3
    The light curves observed by TESS. The pink line represents how much the star was expected to be dimmed by the eclipsing planet and the blue is the actual data. [Gilbert et al. 2020]

    The Host Star

    Understanding the properties of the host star is key in determining the habitability of the planets around it. M-dwarf stars are smaller and dimmer than the Sun but are much more common in the universe. The star that the team found the planets around is called TOI 700. To determine its fundamental properties such as mass and temperature, the team used three different methods in order to validate their results. After using known relations, checking their spectral energy distributions against known spectra, and using spectroscopy, they concluded that the star has an effective temperature of 3,480 K (which is about ⅔ of the Sun’s temperature) and a mass and radius that is about half that of our Sun. They found no flares from the star in the observations over five years, which points to a low amount of magnetic activity, making the system more likely to be habitable.

    Goldilocks (Zone) and the Three Planets

    By analyzing how much the star dimmed as the planets went around it, the team determined that it hosts three planets. From the inner to the outer planet (respectively called TOI 700 b, TOI 700 c, and TOI 700 d), they have radii of 1.01 ± 0.09, 2.63 ± 0.4, and 1.19 ± 0.11 Earth radii. Figure 1 shows how much the planets dim the light of the star. TOI 700 b and d are likely Earth-sized while TOI 700 c is a sub-Neptunian-type planet. TOI 700 d receives about 90% of the energy that the Earth does from the Sun, which places it in the habitable zone of the star. After finding these planets, the team performed tests with many different software packages in order to verify their discoveries. Each of the three planets passed these tests with a false-alarm probability (the probability that the signal is due to something else like instrument noise) of less than 1%. The masses of the planets were determined to be ~1.07 Earth masses, ~7.48 Earth masses, and ~1.72 Earth masses for planets b, c, and d respectively.

    So How Does One Make a Neptune Sandwich…?

    The fact that the largest planet is in the middle of this system is a bit puzzling. Usually planets in a given system have similar sizes — and in the case of our solar system, the inner planets are small and rocky, while the outer planets are larger and gaseous. In this system, the low-density gas planet is sandwiched between the higher-density rocky planets with similar masses. Figure 2 shows the planet in comparison to other systems. The team postulates that this could have come from the two inner planets forming faster and accreting significant gaseous envelopes and the outer one forming more slowly and accreting less gas, then the innermost planet loses its envelope somehow. It is also possible that the middle planet formed outside the outermost planet but migrated inward somehow, but how this could happen isn’t clear. This strange system may be difficult to explain, but it provides a rich laboratory for exploring the formation mechanisms of complex multi-planet systems.

    4
    Figure 2: The TOI 700 planets compared to other known systems. The bottom axis shows flux (or energy received by the planets) compared to Earth’s and the top axis shows distance. [Gilbert et al. 2020]

    Thirty years ago, we did not even know planets could exist around other stars. Now, we know of thousands — and some of those planets are possibly habitable. New exoplanet discoveries like this one are shaking up the field of planetary formation and causing us to rethink our ideas of what stars could host planets and how planets form. As time goes on, new telescopes like James Webb Space Telescope will come online and further expand our understanding of exoplanets. Who knows what kind of weird extrasolar planets we will find next!

    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 11:57 am on February 13, 2020 Permalink | Reply
    Tags: "More Clues to the Environment in Which FRBs Originate?", Astrobites, , , , , FRB 121102 (the repeating FRB “the repeater”)., FRB 191108   

    From astrobites: “More Clues to the Environment in Which FRBs Originate?” 

    Astrobites bloc

    From astrobites

    Feb 12, 2020
    Haley Wahl

    Title: A bright, high rotation-measure FRB that skewers the M33 halo
    Authors: Liam Connor, Joeri van Leeuwen, et. al.
    First Author’s Institution: Anton Pannekoek Institute, University of Amsterdam, Amsterdam, The Netherlands
    1
    Status: Submitted to MNRAS, open access on arXiv

    Fast radio bursts (FRBs) are one of the hottest topics in astronomy right now. First discovered by Dr. Duncan Lorimer in 2007, these intense millisecond-long bursts of radio emission have continued to captivate scientists across the planet because they keep defying our expectations with discoveries like the repeater. Now, with the discovery of an interesting property of a new FRB just outside a major galaxy, we may be getting one step closer to finally solving one of the many puzzles of FRBs.

    More Questions Than Answers

    Our questions about FRBs seem to fall into two categories: What causes the bursts? And how can they be put to use? Each time the community moves toward an answer on one of these questions, a new discovery throws a wrench in it. For example, astronomers thought FRBs were single events but a discovery in 2016 showed that they can actually repeat. This opens new questions, like whether the repeaters and non-repeaters come from the same mechanism. In another case, we thought FRBs only came from dwarf galaxies until one was localized to a massive spiral galaxy. This finding opened more questions about the types of environments that could produce FRBs in very different galaxies. The authors of today’s article present a newly discovered FRB with a very high rotation measure that may give clues to the kind of environment FRBs originate from.

    Understanding Rotation Measures

    When light passes through a medium, it gets Faraday rotated, which is a rotation of the orientation of the light by a magnetic field. How much that light gets rotated as it travels is called its rotation measure (RM). Rotation measure depends on the strength of the magnetic field between us and the source, the density of the material that the light is going through, and the distance to the source. RMs can be used to understand the environment an FRB traveled through. Most of the RMs from FRBs are -100 rad m^-2 to -120 rad m^-2 (a negative RM simply denotes the fact that the magnetic field is pointing away from the observer) but newly discovered FRB 191108 has an RM of almost 500 rad m^-2.

    Caught Between a Galaxy…and Some Interstellar Medium

    FRB 191108 was detected with the Apertif Radio Transient System on the Westerbork Synthesis Radio Telescope in the Netherlands.

    Westerbork Synthesis Radio Telescope, an aperture synthesis interferometer near World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands

    It lies close to the halo of the galaxy M33 (see Figure 1) and is within the intergalactic medium of the Andromeda Galaxy.

    Andromeda Galaxy Adam Evans

    The total observed rotation measure is a combination of the RM from the Milky Way, the RM from the medium between galaxies, and the RM of the host itself. Each of these different components has its own electron density, magnetic field, and thickness so they all contribute to the RM differently. What the authors find from the RM is that it points to an extragalactic contribution of 525 rad m^-2, which would require the magnetic field between galaxies to be 1000x greater than they are. It’s possible that ionized material surrounding the two galaxies could be the cause but because other sources around M33 have RMs of <100 rad m^-2, it is not likely (see Figure 2 for a comparison). Therefore, the RM has to come from somewhere else.

    2
    Figure 1. The location of FRB 191108. The blue cross denotes the most likely location, the red line indicates where the authors believe with 90% certainty the FRB came from, and the circles are the beam size.

    Dense Plasma Environment?

    One of the only explanations the authors find plausible is that the high RM is due to magnetized plasma in the host galaxy. It is possible that the burst originated from an area of very dense, magnetized material. FRBs have been seen to exist in many different environments and some of the RMs of FRBs that have been found point to a high contribution of material from the host galaxy that the burst goes through. FRB 121102 (the repeating FRB, “the repeater”) has an RM that is 100 times greater than FRB 191108 and it has been localized to an environment that is extreme and dynamic. It also has a persistent radio source counterpart to it. The fact that the authors don’t find a radio counterpart to this FRB and don’t find it to repeat means it’s formation environment is likely different from the repeater.

    2
    Figure 2. The RM of sources near M31 and M33. The x-axis shows how far away they are from the galaxies and the y-axis shows the RM. The fact that the RM is so different from the others around the galaxies show that it cannot be caused by plasma around the galaxies (if it was, others along that line of sight would see it).

    Has this discovery pointed to an answer to the question of what kind of environment FRBs originate from, or does the differences between FRB 191108 and FRB 121102 only raise more questions? Only by finding more FRBs will we get closer to answering these questions!

    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 11:38 am on February 12, 2020 Permalink | Reply
    Tags: "Unlocking the secrets of chaotic planetary systems", Astrobites, , , ,   

    From astrobites: “Unlocking the secrets of chaotic planetary systems” 

    Astrobites bloc

    From astrobites

    Feb 11, 2020
    Spencer Wallace

    Title: Fundamental limits from chaos on instability time predictions in compact planetary systems
    Authors: Naireen Hussain, Daniel Tamayo
    First Author’s Institution: Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON

    Status: Accepted for Publication in MNRAS, preprint on arxiv

    It shows up in nearly every field of study – from weather forecasting, to physics, to economics – even sociology – and of course, astronomy. Chaos theory is the study of systems whose seemingly random behavior is the result of an extreme sensitivity to initial conditions. (For an excellent, more in-depth explanation of chaos, check out this astrobite). Chaos is a subject that commonly comes up when trying to understand the long-term stability of planetary systems.

    It turns out that certain arrangements of planets are inherently unstable – that is – if you place them in a certain configuration and let them orbit their star for long enough, the gravitational interactions between the planets will fling some (or sometimes all) of the bodies clear out of the system. Unfortunately, determining how and when this will happen is not possible to work out on paper. Or at least, no one has been clever enough to figure it out yet.

    Fortunately, computers make this problem somewhat tractable. By gradually evolving a collection of massive bodies over many tiny time steps, it is possible to get an incredibly accurate estimate of where and how these bodies will be moving sometime in the future (or the past, for that matter). Given enough computing power, you can simply take a planetary system and evolve it forward in time and see what happens. Does it stay stable? Do any planets get ejected? Using this technique, astronomers can try placing extra bodies in known planetary systems and see if things remain stable. If not, this sometimes can rule out the presence of additional, undetected planets.

    Searching for chaos

    As mentioned above, these types of systems are sometimes chaotic. If so, this means, by definition, that the outcome of whether the system is stable not, and how long it takes to become unstable, is highly sensitive to the initial conditions. The authors of today’s paper want to examine how reliable the estimates of instability timescales from these simulations actually are. If the initial conditions are tweaked just slightly, does this timescale change? And if so, is there an underlying pattern?

    For this study, the authors ran a large suite of N-body simulations of compact, three planet systems (loosely inspired by the well-known TRAPPIST-1).

    A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. NASA

    The TRAPPIST-1 star, an ultracool dwarf, is orbited by seven Earth-size planets (NASA).

    ESO Belgian robotic Trappist National Telescope at Cerro La Silla, Chile

    ESO Belgian robotic Trappist-South National Telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    For each simulation, the orbits and sizes of the planets were varied and the planetary configuration was then evolved for a billion or so orbits to test whether or not the system became unstable. If it did, the timescale for instability was recorded and the same configuration was run again with the initial conditions tweaked ever so slightly to probe the underlying chaotic behavior. If chaos was indeed influencing the outcome, each slight modification to the initial conditions should result in a measurably different instability time.

    2
    Figure 1: The two types of instability timescale distributions found by tweaking the initial conditions of the simulations. In a small number of cases the distribution is sharply peaked (left), while in most cases the instability times follow a log-normal shape.

    After doing this, the authors found that the distribution of instability times for a given configuration of planets fell into two broad categories. This is shown in Figure 1. In some cases, the distribution was very sharply peaked around a single value. Otherwise, the distribution had a log-normal shape.

    The road to dynamical instability

    The difference between these two types of results can be explained by comparing the instability timescale to the Lyapunov timescale, which is how long it takes for chaotic behavior to emerge in a given system. For the sharply peaked distribution, the planets become unstable before chaos sets in. This results in an instability time that is not sensitive to slight changes in the initial conditions. For the broadly peaked distributions, chaos occurs well before the instability. Two example sets of simulations are shown in Figure 2, which demonstrate this difference.

    3
    Figure 2: The Lyapunov time as a function of the instability time for a number of simulations (top). The symbols indicate whether the instability timescales follow a peaked or a log-normal distribution. A representative example of the change in the orbital properties of a planet in each of the two cases is shown at the bottom for the peaked (left) and log-normal (right) results. The ‘shadow trajectories’ indicate how this quantity changes after the initial conditions are slightly tweaked.

    Most interestingly, the log-normal distributions all have a width of ~0.4 dex, regardless of the differences in the initial conditions. The instability time distribution has the same size and shape if the instability time is short or long, or whether the planets are arranged randomly or placed in mean motion resonances. Mean motion resonances occur when the orbital periods are integer multiples of each other. This can act to substantially stabilize or destabilize an orbital configuration, which makes it even more surprising that the instability time distribution shape is not sensitive to this. The remarkable similarity between these distributions across a wide range of configurations is shown in Figure 3. The only requirements here are that the planets start off in a compact configuration and the Lyapunov time be shorter than the instability time.

    4
    Figure 3: The width of the log-normal distribution of instability times for a wide number of simulations. The top panel shows little difference between systems whose planets are arranged randomly and those which begin in mean motion resonances. The bottom panel demonstrates that there is little difference between systems that become unstable quickly and systems which take many millions of orbits for instability to occur.

    It is not terribly surprising that a seemingly random process can give rise to a reproducible pattern. To quote the authors of the paper, “While individual steps in a drunkard’s random walk might be unpredictable, the cumulative effect of many steps approaches a well-defined statistical distribution.” In addition to indicating that the mean instability time in a chaotic system like this can be estimated by running only a small fraction of the simulations required to fill out the entire probability distribution, it hints at a fundamental underlying truth connecting these results produced by an extremely complicated process. The authors do not attempt to speculate on what this similarity tells us about chaotic planetary systems, but it provides a tantalizing clue about the underlying mechanisms that drive this rather abstruse process.

    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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