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  • richardmitnick 1:24 pm on April 30, 2017 Permalink | Reply
    Tags: Astrobites, , , , MLT-mixing length theory, part I, Radiative diffusion, The life and death of stars, The stellar evolution conspiracy   

    From astrobites: “The stellar evolution conspiracy, part I” 

    Astrobites bloc

    Astrobites

    Apr 30, 2017
    Leonardo dos Santos

    Article: Confronting uncertainties in stellar physics II. Exploring differences in main-sequence stellar evolution tracks
    Authors: R. J. Stancliffe, L. Fossati, J.-C. Passy and F. R. N. Schneider
    First author’s institution: Argelander-Institut für Astronomie, University of Bonn
    1
    Status: Published in Astronomy & Astrophysics (February 2016), open access

    Practically all areas of research in astrophysics depend on how well we understand the life and death of stars. Habitability of exoplanets? Yes. Evolution of galaxies? Definitely. The nature of dark matter? Yup. The search for extraterrestrial life? You bet. This is such a crucial component of astrophysics that I decided to discuss the issue in more than one bite (the next one is coming soon). Stars are ubiquitous and drive countless phenomena in the universe. And that is why, at the end of every day, I always ask myself: how much should we trust our understanding of stellar evolution?

    2
    The Pleiades cluster in infrared. This well-known object is a laboratory for testing theories of stellar evolution and structure. Credit: NASA/JPL-Caltech/UCLA

    No need for alternative facts

    Now, I don’t want to sound like a conspiracy theorist or anything, but this is something that is keeping some of us awake at night. Let’s start with the Achilles’ Heel of modern astrophysics: ages of stars. Except for very special cases, stellar ages are particularly tricky to measure because stars change very little throughout their lifetimes. To complicate things further, small changes in the interior structure of a star can produce significant changes in its surface chemical composition. This is why we need our models to be very accurate so that we can have decent estimates of the physical properties of stars (notice that I said “decent”, and not “good”).

    There are many stellar evolution models out there and they are very similar, but it is not clear if any of them are even correct. For starters, it is practically impossible to compute stellar evolution from the first principles of physics, which is why we have to appeal to a series of simplifications and assumptions. Different authors apply different theoretical shortcuts, leading to the emergence of different models.

    Window-shopping stellar evolution models

    Suppose you observed a star identical to the Sun with the Gaia spacecraft and you want to estimate, say, its mass (see Meredith’s bite for a summary on how this estimation can be performed).

    ESA/GAIA satellite

    The authors of today’s paper found that, depending on which one of six available models is chosen, the mass of the star will be between 0.97 and 1.01 solar masses. That is actually a pretty good agreement, which means the models are consistent with each other (see Fig. 1). This is expected, because stellar evolution codes are usually calibrated to reproduce the Sun at its exact mass and age, which we know from other, more precise and accurate methods.

    3
    Figure 1. Evolutionary tracks of a star identical to the Sun (atmosphere temperature in the x-axis, luminosity in the y-axis). The curves are for different models, and the sets of symbols represent different ages. The square corresponds to the observational uncertainties of the Gaia satellite. Notice that all models fall well inside the observational uncertainties, which signals that they are consistent.

    The significant differences start to emerge when we work with stars that have masses and ages that depart from solar values. These are the regimes where our uncertainties about the approximations and assumptions may catch us off-guard. The authors observed that the six stellar evolution models of stars with 3 solar masses are particularly divergent after the main sequence phase (see Fig. 2).

    4
    Figure 2. Similar to Fig. 1, but for a star with 3 times the solar mass. Notice that the models are much more divergent in this case.

    How to mix a giant ball of plasma

    Another issue is that more recent developments in the theory of stellar structure, such as radiative diffusion (which we will discuss in part II), have an impact on the outcomes of models. When the authors tried to re-calibrate these changes with the Sun (using the openly available code MESA), they could not obtain a perfect global fit; it was either a good fit for the solar luminosity and temperature, or its chemical composition, but not all of them at the same time.

    Proposed by Erika Böhm-Vitense in 1958, one widely used approximation to model the convection of material in the atmospheres of stars is known as the mixing length theory (MLT). In a nutshell, the mixing length is the distance a convective cell traverses before dispersing itself. MLT has since been very successful in stellar evolution models, but it comes with a strong caveat: too many free parameters. That means that we observe a well known-star (e.g., the Sun) and calibrate these parameters so that the outcomes of models reproduce what we observe. Free parameters bother us because we don’t know to what extent they are applicable. An alternative to MLT that looks promising is the implementation of 3D hydrodynamical simulations of convection.

    In summary, it turns out that asking “what model should I choose?” is not that useful of a question; what we should actually ask is what are their assumptions and approximations. That way, we are able to analyze if the model is applicable or not to our research given its limitations. In the next part, we will discuss another development on stellar structure that is being heavily discussed by the community, and how it affects stellar age estimates and the search for cosmic siblings.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 1:35 pm on April 26, 2017 Permalink | Reply
    Tags: Astrobites, , , , , The Backwards Discs around Be/X-ray Binaries,   

    From astrobites: “The Backwards Discs around Be/X-ray Binaries” 

    Astrobites bloc

    Astrobites

    Title: Retrograde Accretion Disks in High-Mass Be/X-ray Binaries
    Authors: D. M. Christodoulou, S. G. T. Laycock, D. Kazanas
    First Author’s Institution: Lowell Center for Space Science and Technology, University of Massachusetts Lowell, USA

    Status: Accepted to MNRAS, open access

    1
    Figure 1: The double-discs of a Be/X-ray binary. In the foreground is the Be star, throwing off matter into an equatorial ‘decretion’ disc. Further away, some of that material is pulled towards the neutron star companion, forming an accretion disc. Credit: Gabriel Pérez – SMM (IAC).

    What are Be/X-ray Binaries?

    Stars are like people — they act very different in company from the way they act alone. Interacting binaries are systems in which two stars orbit close together, so that their evolutions are intrinsically linked. They are the hosts of a plethora of astronomical phenomena: type Ia supernovae, millisecond pulsars, cataclysmic variables, common envelopes, and contact binaries are all only possible in interacting binary systems.

    Among these systems are X-ray binaries.

    2
    Representation of an X-ray binary. (Credit: NASA/R. Hynes)

    These are accreting binaries — systems in which material is flowing from the less massive star (the ‘donor’) onto the more massive star (the ‘accretor’). In the case of X-ray binaries, the star on which the matter is falling is a neutron star or black hole. The infalling material spirals towards the accretor, forming an accretion disc.

    3
    Ring Around a Suspected Black Hole in Galaxy NGC 4261.
    Date 29 October 1995
    Source HubbleSite: gallery, release.
    Author L. Ferrarese (Johns Hopkins University) and NASA

    Friction in the disc heats it up until it is hot enough to produce the bright X-rays that name this type of system. Similar accretion discs are seen throughout astronomy — in cataclysmic variables, active galactic nuclei, and protoplanetary systems, to name a few varieties — but are pretty hard to model, so there’s a great deal of interest in exploring the different varieties they come in.

    In the standard X-ray binary model, material is pulled from the atmosphere of the donor star by the accretor’s gravity. Today’s paper is about Be/X-ray binaries, a special case in which the donor star is a Be-type star and the transferred material is a stellar wind. A Be star is a star spinning so fast that it throws some of its own matter off into space. Put a Be star into orbit around a neutron star, and some of that expelled matter will fall towards the neutron star — and voilà, you have your accretion. The orbits of these systems are often elliptical, meaning that at some points in their orbit the stars are close together (resulting in a higher accretion rate and an extra spurt of X-ray emission) and at others they are further apart (causing a dip in the X-ray emission).

    If the accretor is a neutron star with a magnetic field, we also see pulses in the brightness of the system caused by the spinning of the neutron star’s magnetic field. This means we can measure how quickly the neutron star spins on its axis — its ‘spin period’. Pulsars spin quickly: the fastest stars in todays paper spin on their axes once every few seconds; the slowest, once every 30 minutes.

    Slowing Down or Speeding Up?

    4
    Figure 2: Both spinning-up and spinning-down neutron stars show the same dependence between the rate of change in their spins and how fast they are already spinning — the only difference is that some are speeding up by that amount, and some are slowing down by the same amount. This is Figure 2 from today’s paper.

    The author’s of today’s paper compared results from a catalogue of Be/X-ray binaries in the Small Magellanic Cloud, all of which had their spin periods measured continuously between 1997 and 2014.

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

    Over that time, there were 53 binaries in which the spinning of the neutron star noticeably changed — either increasing or decreasing in period.

    For those neutron stars whose spin is accelerating, there is a commonly accepted explanation. When the infalling material lands on the neutron star, it transfers any angular momentum it has to the star and spins it up. However, in the sample the authors were studying, they were surprised to find that nearly half of the neutron stars were ‘spinning down’ — their spin was decelerating. The surprise comes from the fact that we don’t know of any mechanism that can change the angular momentum of these stars as efficiently as the accretion process.

    To investigate further, the authors compared the spin-up and spin-down rates as a function of the spin period of the neutron stars. You can see their results in Figure 2, in which is plotted the magnitude of the spin up/down against spin period. They found that both groups seem to follow the exact same pattern, but in opposite directions. A neutron star with a long spin period is likely to be either accelerating or decelerating sharply, whereas a neutron star with a short spin period is likely to have a much more gradual acceleration or deceleration. You could fit the same straight line through both populations in Figure 2.

    The appearance of the same pattern in both groups of systems implies that the two populations must be linked, and that the processes driving their evolutions must be similar. This would mean that the accretion driving the spin-up in some systems must also drive the spin-down in others. The only way this could work is if those spinning-down systems have accretion discs spinning backwards — accretion discs that rotate in the opposite direction to the spin of the neutron star. The term for this is ‘retrograde’.

    Such backwards-spinning accretion discs have been proposed before for a few individual systems, but never for Be/X-ray binaries. If it’s true, it could be very interesting for models of how these systems form and evolve, particularly if — as the authors suggest at the end of their paper — the accretion discs can switch between rotating with and against the neutron star. Today’s paper was only a short letter introducing the idea; it will be exciting to see where this goes next!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 2:56 pm on April 25, 2017 Permalink | Reply
    Tags: Astrobites, , , , Spinning up massive classical bulges in spiral galaxies   

    From astrobites: “Spinning up massive classical bulges in spiral galaxies” 

    Astrobites bloc

    Astrobites

    Apr 25, 2017
    Sandeep Kumar Kataria – guest writer

    Title: Spin-up of massive classical bulges during secular evolution
    Authors: Kanak Saha, Ortwin Gerhard, and Inma Martinez-Valpuesta
    First Author’s Institution: Inter-University Center for Astronomy and Astrophysics, Pune
    1
    Status: Accepted for publication in Astronomy & Astrophysics, open access

    Introduction:

    The mass of spiral galaxies is mainly distributed in three components: the classical bulge (ClB), disc, and surrounding dark matter halo.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    Classical bulges are the central building blocks of many early-type spiral galaxies (see the Astrobites Guide to Galaxy Types). These bulges might have formed as a result of collisions between galaxies in the early universe or various other processes mentioned in this paper. It is believed that initially the motion of stars in ClBs is disordered, so the ClB does not rotate. The authors of this paper see an interesting problem to ponder: in the present day, there is an observed net rotation of stars in classical bulges. The origin of this rotation is still to be understood in detail.

    One of the authors of this paper has explained in earlier work that low-mass classical bulges spin up by absorbing angular momentum from galactic bars. The bar has a pattern speed, which is a measure of the collective rotation of a family of orbits of stars in the bar. Angular momentum exchange from the bar mainly occurs at resonances in the disc. These are locations where the difference between disc’s rotation speed and the bar pattern speed have specific ratios with radial oscillations of the stars in the disc. These resonances can be thought of as analogous to resonances in an organ pipe, the natural frequency of which corresponds to waves with wavelengths which match the length of the organ pipe. Let’s see how the authors approach the solution of the rotation problem in ClBs.

    Experiments with galaxy models using computers:

    The authors of this paper try to explain net rotations in Massive ClBs using N-Body simulations.

    2

    First, models of galaxies having non-rotating classical bulges of different masses and sizes are generated using well known techniques such that these models are not unstable. One of the well known classical parameters of local stability is the Toomre Parameter. This parameter measures the ratio between inward gravitational pull on stars at a particular point, and the radial motions of stars at that point. If these motions are sufficiently strong, the gravitational pull will be insufficient to overcome them and the disc will be locally stable. All the models, after evolution, form bars of different sizes according to the initial value of the Toomre parameter. Further, the point of interest lies in understanding how these bars transfer angular momentum to ClBs.

    Studying Bulge Kinematics from experiments:

    4
    Figure 1a. Top row – surface density maps of the model with the highest mass ClB at different times during its evolution. Second to fourth rows – line-of-sight velocity (left) and velocity dispersion (right) maps at different times. These images are taken at 90° projection (edge-on view) and the major axis of the bar is aligned with the x-axis. Clear signatures of rotation are seen at 4 Gyr. The colour bar at the top represents density, middle the velocity, and bottom the velocity dispersion.
    Figure 1b. Rotation, velocity dispersion, and local V/σ radial profiles for the four ClBs in the models.

    The authors notice changes in orbital configuration due to angular momentum transfer by the bar. From Figure 1a it can be noticed that the rotational component in the outer part of the bulge increases over time. It can also be seen that the central part of bulge becomes ‘hot’ and slightly rounder. Here ‘hot’ means that orbits of stars become more disordered and their velocity dispersion (σ) increases. Figure 1b shows radial profiles of rotation and dispersion of stars in the bulge at 4 Gyr for a few of the simulated models. It can be deduced that ClBs rotate faster in their outer parts. However, comparing simulated rotation data of ClBs with observations is no easy task: observational rotation data contains stars both in bulges and bars and distinguishing which they belong to at a single moment in time is challenging.

    3
    Figure 2a. Top row: distribution of bulge stars with frequency (Ω − ΩB)/κ at different times throughout the secular evolution in the model with the lowest bulge mass. Bottom row: net change in the angular momentum of the selected stars with respect to the previous time. The vertical dotted lines indicate the most important resonances (from left to right): −1:1, 4:1, 3:1, 5:2, and 2:1. As time progresses, more stars are trapped by the 2:1 resonance of the bar with the stellar disc. However, most of the angular momentum transfer occurs through the 5:2 resonance.
    Figure 2b. Here the top and bottom rows represent same entities as in the previous figure but for the models with the highest mass ClBs. As with the low mass Classical bulges most of the angular momentum transfer occurs via the 5:2 resonance.

    The Spin-up process in Massive Classical Bulges:

    After simulating galaxy models with various types of ClBs, the authors conclude that specific angular momentum (angular momentum per unit mass) transfer by the bar is the same for ClBs with low and high mass. Most of the angular momentum transfer from the disc to the bulge occur at particular locations (resonances) which are shown in Figures 2a and 2b. This phenomenon lead to density wakes (alignments of stars in the bulge with the bar) in the bulge. In the simulations density wakes are not so aligned with the bar in the low-mass ClBs but are completely aligned with the high mass ClBs by the end of simulation. The authors also find that outer parts of the bulge experience significant amount of rotation. In addition, the orbits in low-mass bulges are well-ordered, but the ones in high-mass bulges are more disordered. At the end of the simulation all models have a bar with a ‘box’ shape, suggesting that composite bulges (ClB + Boxy Bar) should be common in galaxies. Finally the authors conclude that massive ClBs, like low mass ClBs, are affected by angular momentum exchange with the bar. The spin up process is more prominent when the bar is larger than the ClB.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 9:23 am on April 24, 2017 Permalink | Reply
    Tags: Astrobites, , , , Breaking Planet Chains and Cracking the Kepler Dichotomy, , Kepler Dichotomy, , Planetary migration   

    From astrobites: “Breaking Planet Chains and Cracking the Kepler Dichotomy” 

    Astrobites bloc

    Astrobites

    Apr 24, 2017
    Michael Hammer

    Title: Breaking the Chains: Hot Super-Earth systems from migration and disruption of compact resonant chains
    Authors: Andre Izidoro, Masahiro Ogihara, Sean N. Raymond, Alessandro Morbidelli, Arnaud Pierens, Bertram Bitsch, Christophe Cossou, Franck Hersant
    First Author’s Institution: Laboratoire d’astrophysique de Bordeaux, University of Bordeaux
    1
    3

    Status: Submitted to MNRAS [open access]

    To migrate, or not to migrate? That is the question. Of course, since planets are not Shakespearean characters, they should not have a choice! When a planet forms in a disk, it creates two spiral waves: a weaker one ahead of the planet that drags it forward (sending the planet outwards), and a stronger one behind the planet that pulls it backwards (sending the planet inwards). Ultimately, every planet should migrate inwards and in most cases, end up much closer to its star than where it formed.

    When planets in the outer disk migrate inwards faster than planets closer in, they start to catch up to each other. As these planets get closer together, they eventually become gravitationally locked into resonance: pairs of orbits where the outer planet takes exactly twice as long (or another integer ratio such as 3-to-2, etc.) to complete an orbit around its star as the inner one. Once this happens, the planets migrate together, maintaining that 2-to-1 ratio. In systems with many rocky planets, the third one will follow suit and fall into a resonance with the second planet, as will the fourth with the third, and so on. Eventually, the system will have a long chain of up to 10 resonant rocky planets tightly packed in the inner part of the disk!

    Yet even though migration is supposed to be inevitable, only about 5% of the planetary systems discovered by the Kepler mission are actually in this setup (TRAPPIST-1 is the most famous).

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

    The other 95% are not, many of which because they only have one planet. Today’s paper, led by Andre Izidoro, attempts to explain these discrepancies by suggesting that all systems migrate into resonant chains, but not all of them stay in resonant chains!

    Two-Phase Setup

    Izidoro et al. study this problem by conducting two-phase N-body simulations of 120 hypothetical planetary systems with 20 to 30 rocky planets for 100 Myr. These planets start out with 0.1 to 4.5 Earth masses and are spread out evenly in the outer disk beyond 5 AU.

    In phase one (0 to 5 Myr), the planets may migrate due to the presence of a gaseous protoplanetary disk. Meanwhile, the disk also keeps the planets on flat, circular orbits by damping the planets’ eccentricities and inclinations.
    In phase two (5 to 100 Myr), the planets can no longer migrate since the disk has dissipated away. However, they are free to develop eccentric and inclined orbits since they are now controlled by interactions with each other instead of interactions with the disk.

    Compact, but not too compact

    Izidoro et al. find that all of their planetary systems migrate into compact resonant chains within 1.5 Myr, safely less than the disk’s lifetime of 5 Myr. Many of these systems (40%) then survive as resonant chains for the entire 100 Myr simulation.

    However, some systems (60%) become too compact (see Figure 1). In particular, the ones that are too compact with higher mass planets become unstable after the disk fades away! The resonant chains then collapse as some of the planets eject and the rest spread farther apart. As they spread out, the surviving planets’ orbits also become more eccentric and inclined.

    2
    Figure 1. Two example resonant chains after phase one. The first system (left) will survive phase two (without the disk). The second system (right) will become unstable because it has more planets too close together. Some of the surviving planets will develop inclined orbits, making them less likely to transit. Adapted from Figs. 2 and 3 of the paper.

    Single-Planet Imposters

    In order to compare their results with actual exoplanet systems discovered by the Kepler Mission, Izidoro et al. must determine what fraction of their planets can transit (and be “detected” by Kepler).

    Planet transit. NASA/Ames

    NASA/Kepler Telescope

    They find that in the stable resonant chains, Kepler can detect 3 or more planets in 66% of these systems. On the other side in the unstable systems, the inclined orbits from the instabilities make it so that Kepler can only detect 1 planet in 78% of these systems, even though over 90% of the unstable systems still have multiple planets.

    Explaining the Kepler Dichotomy

    One of the defining features of Kepler’s planets is the large number of systems with only one transiting planet. Naturally, we expected that Kepler would not be able to find all of the planets in each of its systems since planets at large separations from their star that do not line up with our line-of-sight will not transit. However even with this bias, the fact that there are so many more single-planet systems than two-planet systems (see Figure 2) suggests that Kepler systems belong to a dichotomy: roughly 50% of all systems have just one planet (including non-transiting ones) and 50% have many planets (5+ for small stars). Such a high fraction of single-planet systems is a huge surprise, given how many planets exist in our own solar system.

    However, the two populations of planetary systems in this study offer an explanation for the Kepler dichotomy that would imply these single planets are not so lonely. Izidoro et al. calculate that if no more than 25% of all planetary systems are compact resonant chains (with the rest being unstable systems), this distribution of systems can match the high fraction of systems with just one transiting planet in the Kepler dichotomy — even though nearly all of these systems would have multiple planets.

    2
    Figure 2. Comparison of Kepler’s planetary systems to this paper’s planetary systems. In the Kepler sample (green), the vast majority of systems have only one transiting planet. The unstable systems in this paper (blue) would have even more single-transit systems, while the stable resonant chains (red) have a lot fewer. A proper balance between these two (90% unstable, 10% stable — gray) matches the Kepler dichotomy pretty well. Fig. 15 of the paper.

    Why so unstable?

    Izidoro et al. expect that in reality, roughly 5% of all planetary systems are stable resonant chains (since this is the fraction found by Kepler), which is consistent with their upper limit of 25% they need to explain the dichotomy. Even though the authors find that 40% remain stable in their study, they suspect that simulations with a more realistic protoplanetary disk would lead to many more systems going unstable. Nonetheless, the authors caution that their model remains incomplete until they find a reason for ~95% of Kepler’s systems becoming unstable at some point in their history.

    It may also be the case that not all systems migrate into resonant chains to begin with, or even that planets do not migrate as easily as this study presumes. For now, we can still take solace in knowing that at least some of Kepler’s single-planet systems have non-transiting companions that they can orbit with for billions of years.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 April 21, 2017 Permalink | Reply
    Tags: 2 furious, 2 slow, Astrobites, , , , , MASSIVE survey, SAMI galaxy survey   

    From astrobites: “2 slow, 2 furious” 

    Astrobites bloc

    Astrobites

    Apr 21, 2017
    Paddy Alton

    Titles:
    1. The MASSIVE survey – VII. The relationship of Angular Momentum, Stellar Mass and Environment of Early-Type Galaxies
    2. The SAMI galaxy survey: mass as the driver of the kinematic morphology – density relation in clusters

    Authors:
    1. Melanie Veale, Chung-Pei Ma, Jenny E. Greene, et al.
    2. Sarah Brough, Jesse van de Sande, Matt S. Owers, et al.

    First Authors’ Institutions:
    1. University of California, Berkeley, USA

    2. University of New South Wales, Australia

    Statuses:
    1. Submitted to Monthly Notices of the Royal Astronomical Society [open access]
    2. Submitted to the Astrophysical Journal [open access]

    Introduction

    Scientific papers are a bit like buses. Sometimes you wait for ages waiting for one to take you where you want to go, then – surprise, surprise – two come along at once. This is, of course, a fundamental physical law, to which even astrophysicists are not immune.

    In today’s article I’m going to break with tradition a little bit and highlight not one, but two papers, released weeks apart and with similar goals. This happens reasonably often, principally because if the science is both exciting and possible, chances are more than one team are looking into it! It’s always interesting to see independent groups take on the same question – and of course, the replicability of results is at the core of the scientific method. So for those reasons, and in the interests of fairness, let’s look at two takes on the origin of fast and slow rotating elliptical galaxies.

    Fast and Slow Rotators

    In the last decade the terms ‘fast rotator’ and ‘slow rotator’ entered astrophysical parlance as detailed studies revealed important differences among nearby galaxies. At first sight all elliptical galaxies look much alike, being more-or-less featureless red-ish blobs (see figure). However, a closer look reveals that they exhibit two quite distinct types of kinematic behaviour (the term kinematic in this context refers to the movement of stars within a galaxy, in other words its internal motions). This important detail has been highlighted by Astrobites before.

    The terminology here is not particularly imaginative: the principal difference between fast and slow rotators is, well, that the former rotate faster than the latter. But let us go into a bit more depth. Galaxies are collisionless systems, meaning that the gulf separating stars is sufficiently vast relative to their size that head-on collisions never happen in practice. Instead, all interactions are through gravity; stars whip around their host galaxy, their motions governed by its gravitational potential well. The orbits of the stars can be correlated, so that they are mostly orbiting around the same axis and in the same direction – or messy, with disordered orbits. Moreover, while all closed orbits are ellipses, there’s a big difference between a nearly circular orbit (like the Earth going round the sun) and a highly elongated orbit (like that of a long-period comet). These extremes are sometimes respectively referred to as tangential and radial orbits.

    If the orbits of stars in a galaxy are mostly correlated and tangential, the galaxy ends up as a flattened, more oblate rotating system. By contrast, disordered radial orbits give you blob-like systems without much rotation. In the first case, we might say that the system is ‘rotation supported’ (it doesn’t collapse down to a point under its own gravity because it’s rotating and can’t shed its angular momentum) and in the second that it is ‘pressure supported’ (stars falling in towards the centre are balanced by stars that have already passed through the centre and are now travelling outwards). This gets to the crux of the matter: most elliptical galaxies are rotation-supported fast rotators, but a significant fraction (about 15%) are pressure-supported slow rotators. The stark difference in their kinematics has led to suggestions that despite their apparent similarities, an alternate formation channel is required to create slow rotators.

    Today’s papers

    In order to get to the bottom of this, the two teams conducted similar investigations. Both used data from large surveys of many galaxies, the MASSIVE survey and the SAMI galaxy survey respectively. Both surveys provide detailed spectroscopy of many galaxies – large samples are necessary since the aim is to draw statistical conclusions about the population of slow rotator galaxies as a whole. From this data, the kinematics of each target can be inferred (I explained how that works in some detail in a previous article, but it’s not essential to recap all that here).

    Encouragingly, both studies hold some conclusions in common. As was already believed to be the case, both find that slow rotators are preferentially found among the most massive galaxies. Both teams looked at the effect of galaxy environment (i.e. whether the galaxy is isolated or contained in a cluster with many nearby neighbours). Massive galaxies do tend to be more commonly found in clusters, so given the dependence on mass already established such a trend must exist. What’s important is that when mass is controlled for there is no additional dependence on environment: both teams concur on this point.

    Conclusions

    Galaxies tend to grow via a series of mergers – collisions – between smaller galaxies, a process that takes place faster in dense environments where the chance of encountering another galaxy is much higher. This is the explanation for the point made above, that galaxies in clusters tend to be more massive than their isolated counterparts.

    In the past it has been suggested that slow rotators might form due to a major collision between two similar sized galaxies, a highly disruptive event that would of course tend to leave behind a particularly massive galaxy. This kind of event would be much more common in the centre of a cluster of galaxies. However, neither of the studies presented here find strong evidence for a ‘special’ formation channel like this!

    It’s certainly true that slow rotator galaxies tend to be particularly massive, but they don’t seem to care how they were put together (i.e. by many minor mergers or one big major merger): whether minor or major, galaxy mergers will tend to add mass and (usually) decrease the angular momentum of a galaxy. The more mergers that occur (i.e. the more massive a galaxy gets), the slower it will tend to rotate. In other words, fast rotators that grow large enough will eventually transition to become slow rotators instead.

    See the full article here .

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    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 11:22 am on April 20, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Cradles of Massive Stars   

    From astrobites: “Cradles of Massive Stars” 

    Astrobites bloc

    Astrobites

    Title: Thermal Feedback in the High-mass Star and Cluster Forming Region W51
    Authors: Adam Ginsburg, Ciriaco Goddi, J.M. Diederik Kruijssen, John Bally, Rowan Smith, Roberto Galván-Madrid, Elisabeth A. C. Mills, Ke Wang, James E. Dale, Jeremy Darling, Erik Rosolowsky, Robert Loughnane, Leonardo Testi, Nate Bastian
    First Author’s Institution: National Radio Astronomy Observatory, Socorro, NM 87801 USA

    Status: Accepted to the Astrophysical Journal, open access

    1
    Figure 1. W51 as seen by the radio observatories ALMA and VLA. Images from radio observations are ‘false color’, meaning that the colors represent light that cannot be seen with naked eyes. Color scheme: blue is the carbon monoxide (CO) line, orange is the methanol (CH3OH) line, purple is the cyanoacetylene (HC3N) line, green is the radio continuum, and the white haze is free-free emission of ionized gas. [Figure 1 of original paper.]

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

    NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    Today let’s talk about massive stars! My favorite view of massive stars is the Hubble image of the star cluster R136 in the Large Magellanic Cloud.

    3
    Astronomers using the unique ultraviolet capabilities of the NASA/ESA Hubble Space Telescope have identified nine monster stars with masses over 100 times the mass of the sun in the star cluster R136. This makes it the largest sample of very massive stars identified to date.

    All the blue shining spots in this picture are massive stars, with masses up to hundreds of solar masses that are million times brighter than the sun! Massive stars bring beauty to our night skies, as well as structures to our Universe. The Hubble image shows massive stars in their magnificent adulthood. But have you ever wondered what they looked like when they were still babies?

    Indeed we know very little about their babyhood because baby massive stars are very far away and are usually blocked by opaque dust. To study their births we need observations at longer wavelengths. By looking at infrared (IR) wavelengths, we can study the dust that is heated by newly formed stars, which would provide clues to the embedded stars. By studying radio wave line emission, we can see and trace the dense gas that comes before star formation. Radio free-free continuum shows the compact ionized (HII) regions around young stars. Today’s paper does all these, looking into the high-mass star forming region W51 (shown in Figure 1) using ALMA. ALMA’s extraordinary angular resolution never ceases to amaze me. Today’s observations were done at ~0.2″ resolution, it is equivalent to telling two quarters apart at a distance of ~25 km (about ten standard airport runways placed back-to-back)!

    The paper looks at three baby massive stars in W51, namely e2, e8, and North (Figure 1). These objects were chosen because of their strong star formation and the gas clouds have not been destroyed by supernova explosions, which are the key ingredients for understanding high-mass star formation. While the authors uncovered a wealth of information through their observations in the paper, here we focus on two main aspects: the temperature and ionization structures around the baby massive stars.

    4
    Figure 2. Temperature map around the hot core e2. This map was created using the molecular emission lines of methanol around the source. We see that the baby high-mass star heats up a large volume with a radius about 5000 AU. [Figure 6a of original paper.]

    Figure 2 shows the temperature map of the dense gas around the baby massive star e2, created by modeling eight methanol emission lines. The main takeaway is that baby massive stars heat up a large volume of surrounding gas in their early formation phase, preventing gas from fragmenting and keeping the reservoir of gas available for star formation. The contour (blue line) in the temperature map encloses the region above 350 K, encompassing a region with radius ~5000 AU. This temperature is much higher than the ~10 K typically observed in interstellar gas.

    5
    Figure 3. Image showing the highly excited warm molecular gas (colors) and the free-free radio emission from ionized gas (contours) around e2. The legend shows the nature of different colors. The absence of enhanced heating around the ionized region suggests that ionizing radiation has little effect on the dense molecular gas. [Figure 15a of original paper.]

    What about the ionization structure? Figure 3 shows the warm molecular gas (colors) and the ionized gas (contours) around e2. Again the bright emission in colors shows that the baby stars are responsible for heating up the nearby dense gas. There are two keys features:

    There is no enhanced heating of dense gas (brighter colors) around the ionized region (contours). The authors conclude that ionizing radiation from already-formed massive stars has little effect on the star-forming gas;
    The bright dust continuum emission (left blue blob) predicts strong ionizing radiation from the embedded baby stars, but the corresponding free-free emission (white contours) is not observed. The authors proposed an explanation: rapid accretion onto the growing stars bloats them and reduces their surface temperature, making them too cold to emit ionizing radiation. This is a big deal! The working of simultaneous gas infall and outward radiation feedback is extremely hard to model even with simulations and supercomputers. Today’s paper presents the first observational insight on what actually happens to growing massive stars!

    Today’s paper is a pedagogical piece showcasing how bright scientists and next-generation observatories translate into new insights for future observations and simulations. These insights are necessary for us to understand how the beautiful massive cluster R136 came to be. Indeed, we expect the active star formation in the early Universe behaved similarly to within forming massive clusters. Understanding how massive star clusters form therefore provides a unique basis to connect the cosmic history of star formation.

    p.s. The first author of today’s paper is also an active developer in astronomy softwares, check out his Github page!

    See the full article here .

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    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 11:04 am on April 20, 2017 Permalink | Reply
    Tags: Active Cryovolcanism on Europa?, Astrobites, , ,   

    From astrobites: “Active Cryovolcanism on Europa?” 

    Astrobites bloc

    Astrobites

    Apr 20, 2017
    Joseph Schmitt

    Title: Active Cryovolcanism on Europa?
    Authors: William B. Sparks, Britney E. Schmidt, Melissa A. McGrath, Kevin P. Hand, John .R. Spencer, Misty Cracraft, and Susana E. Deustua
    First Author’s Institution: Space Telescope Science Institute

    Status: Submitted, open access

    Europa, one of Jupiter’s Galilean moons, is one of the most exciting places in the search for alien life in our solar system, rivaling both Mars and Saturn’s moon Enceladus.

    1
    Europa

    2
    Enceladus

    Underneath a 15-25 km surface layer of ice, Europa very likely has a thick (~100 km) ocean of salty water with a rocky seafloor. Chemical reactions on the icy surface caused by high-energy particles from Jupiter’s radiation belts could provide some of the essential ingredients for life, but only if this material could somehow reach the liquid water beneath it. These geological properties make Europa a prime candidate for potential alien life.

    Cryovolcanism and Cryogeysers

    Cryovolcanoes are similar to the volcanoes here on Earth with subsurface “magma” being expelled from the planet’s interior and then solidifying on the surface. For cryovolcanoes though, this “magma” is liquid water instead of molten rock. Both types of volcanism require internal heat sources. On Earth, this primarily comes from the residual heat of Earth’s formation and radioactive decay. For the icy moons of the outer solar system, the main expected heat source is from tidal heating. There are many suspected or possible examples of cryovolcanism in the solar system on many different bodies: Ceres, Europa, Enceladus, Titan, Miranda, Triton, Pluto, Charon, and Quaoar, although none have been definitively confirmed.

    Cryogeysers are similar in some aspects to cryovolcanoes (and are sometimes included in the definition of “cryovolcanism”). However, cryogeysers expel volatile gases (with some solid particles) instead of a liquid magma/lava. Examples of this are known on Enceladus, Triton, and Mars’ ice caps. Hubble observations from 2013 and 2014 also suggested the existence of cryogeysers on Europa by finding what appears to be plumes of water vapor rising above Europa’s surface, although this was not seen in every observation. This created uncertainty as to whether the signal was truly there or whether the plumes are just variable.

    New Evidence of Cryovolcanism on Europa

    The authors of the article in today’s Astrobite did follow-up Hubble observations in early 2016. They also observed a potential plume of water vapor in the same location as the previous observations, although they were still unable to definitively confirm it. There are two popularly supported mechanisms for creating these plumes: an explosion of dissolved gases in a pressurized liquid after the outside pressure has been removed and the expansion of ice when a trapped body of water freezes that then breaks out of its enclosure.

    The location of the new plume candidate also matches the location of a thermal hotspot on Europa’s surface that was first detected by the Galileo spacecraft in 1999, the cause of which remains unknown. The hotspot could be due to the ice layer being very thin at this point (~2 km) or potentially a large reservoir of liquid water trapped in the middle of the ice layer. Another source could be from internal heating, such as tidal heating that is somehow focused into that location or plumes of warm water underneath the icy surface. Alternatively, the thermal hotspot could just hold onto its heat better than the rest of the surface (i.e., it has a higher thermal inertia). This could be caused by different materials being deposited on the surface of the hotspot due to local vents/cryogeysers. The potential plume and hotspot, however, do not correspond to any surface features that might help explain their source.

    3
    Figure 1: A zoom-in of Europa’s surface. Panel (a): the green ellipse signifies the location of a 2014 candidate plume, while the cyan ellipse signifies the location of the 2016 candidate plume from this new paper. Panel (b): a contour heat map of the same region of Europa showing the hotspot located at the same position as the two candidate plumes from Panel (a).

    Conclusions

    While the authors were unable to definitively prove the existence of a water vapor plume, they did produce strong evidence of its existence, tying it together with a previous plume candidate and a thermal hotspot that appears in the same location. This is hard to explain away as a mere coincidence. The authors believe that this likely means that the icy surface layer is particularly thin in that region (~2 km thick instead of the usual 15-25 km) or that material has been deposited on that region from a local cryogeyser/vent. These observations add to the growing body of evidence that Europa may have the conditions necessary to host life.

    See the full article here .

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    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 12:36 pm on April 18, 2017 Permalink | Reply
    Tags: , Astrobites,   

    From AAS NOVA: “Samples and Statistics: Distinguishing Populations of Hot Jupiters in a Growing Dataset” 

    AASNOVA

    American Astronomical Society

    astrobites

    18 April 2017
    Jamila Pegues

    Title: Evidence for Two Hot Jupiter Formation Paths
    Authors: Benjamin E. Nelson, Eric B. Ford, and Frederic A. Rasio
    First Author’s Institution: Northwestern University

    Status: Submitted to AJ, open access

    5
    Figure 1: A gorgeous artist’s impression of a hot Jupiter orbiting around its host star. [ESO/L. Calçada]

    Frolicking Through Fields of Data

    The future of astronomy observations seems as bright as the night sky … and just as crowded! Over the next decade, several truly powerful telescopes are set to launch (read about a good number of them here and also here).

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    NASA/TESS

    Giant Magellan Telescope, Las Campanas Observatory, to be built some 115 km (71 mi) north-northeast of La Serena, Chile

    TMT-Thirty Meter Telescope, proposed for Mauna Kea, Hawaii, USA

    ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile


    LSST Camera, built at SLAC



    LSST telescope, currently under construction 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.

    NASA/ESA/CSA Webb Telescope annotated

    NASA/WFIRST

    FAST radio telescope, now operating, located in the Dawodang depression in Pingtang county Guizhou Province, South China

    That means we’re going to have a LOT of data on everything from black holes to galaxies, and beyond — and that’s in addition to the huge fields of data from the past decade that we’re already frolicking through now. It’s certainly far more data than any one astronomer (or even a group of astronomers) wants to analyze one-by-one; that’s why these days, astronomers turn more and more to the power of astrostatistics to characterize their data.

    The authors of today’s astrobite had that goal in mind. They explored a widely-applicable, data-driven statistical method for distinguishing different populations in a sample of data. In a sentence, they took a large sample of hot Jupiters and used this technique to try and separate out different populations of hot Jupiters — based on how the planets were formed — within their sample. Let’s break down exactly what they did, and how they did it, in the next few sections!

    Hot Jupiters Are Pretty Cool

    First question: what’s a hot Jupiter, anyway?

    They’re actually surprisingly well-named: essentially, they are gas-giant planets like Jupiter, but are much, much hotter. (Read all about them in previous astrobites, like this one and this other one!) Hot Jupiters orbit perilously close to their host stars — closer even than Mercury does in our own Solar System, for example. But it seems they don’t start out there. It’s more likely that these hot Jupiters formed out at several AU from their host stars, and then migrated inward into the much closer orbits from there.

    As to why hot Jupiters migrate inward … well, it’s still unclear. Today’s authors focused on two migration pathways that could lead to two distinct populations of hot Jupiters in their sample. These migration theories, as well as what the minimum allowed distance to the host star (the famous Roche separation distance, aRoche) would be in each case, are as follows:

    Disk migration: hot Jupiters interact with their surrounding protoplanetary disk, and these interactions push their orbits inward. In this context, aRoche corresponds to the minimum distance that a hot Jupiter could orbit before its host star either (1) stripped away all of the planet’s gas or (2) ripped the planet apart.
    Eccentric migration: hot Jupiters start out on very eccentric (as in, more elliptical than circular) orbits, and eventually their orbits morph into circular orbits of distance 2aRoche. In this context, aRoche refers to the minimum distance that a hot Jupiter could orbit before the host star pulled away too much mass from the planet.

    The authors defined a parameter ‘x’ for a given hot Jupiter to be x = a/aRoche, where ‘a’ is the planet’s observed semi-major axis. Based on the minimum distances in the above theories, we could predict that hot Jupiters that underwent disk migration would have a minimum x-value of x = aRoche/aRoche = 1. On the other hand, hot Jupiters that underwent eccentric migration would instead have a minimum x-value of x = 2aRoche/aRoche = 2. This x for a given planet is proportional to the planet’s orbital period ‘P’, its radius ‘R’, and its mass ‘M’ in the following way:

    And this x served as a key parameter in the authors’ statistical models!

    Toying with Bayesian Statistics

    Next question: how did today’s authors statistically model their data?

    4
    Figure 2: Probability distribution of x for each observation group, assuming that each hot Jupiter orbit was observed along the edge (like looking at the thin edge of a DVD). The bottom panel zooms in on the top one. Note how the samples have different minimum values! [Nelson et al. 2017]

    Short answer: with Bayesian statistics. Basically, the authors modeled how the parameter x is distributed within their planet sample with truncated power laws — so, x raised to some power, cut off between minimum and maximum x values. They split their sample of planets into two groups, based on the telescope and technique used to observe the planets: “RV+Kepler” and “HAT+WASP”. Figure 2 displays the distribution of x for each of the subgroups.

    The authors then used the Markov Chain Monte Carlo method (aka, MCMC; see the Bayesian statistics link above) to explore what sort of values of the power laws’ powers and cutoffs would well represent their data. Based on their chosen model form, they found that the RV+Kepler sample fit well with their model relating to eccentric migration. On the other hand, they found evidence that the HAT+WASP sample could be split into two populations: about 15% of those planets corresponded to disk migration, while the other 85% or so corresponded to eccentric migration.

    Remember that a major goal of today’s authors was to see if they could use this statistical approach to distinguish between planet populations in their sample … and in that endeavor, they were successful! The authors were thus optimistic about using this statistical technique for a much larger sample of hot Jupiters in the future, as oodles of data stream in from telescopes and surveys like KELT, TESS, and WFIRST over the next couple of decades.

    Their success joins the swelling toolbox of astrostatistics … and just in time! Telescopes of the present and very-near future are going to flood our computers with data — so unless we’re willing to examine every bright spot we observe in the sky by hand, we’ll need all the help from statistics that we can get!

    See the full article here .

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  • richardmitnick 12:10 pm on April 18, 2017 Permalink | Reply
    Tags: Astrobites, , , Australia, , , , Live fast die young: quiescent galaxies in the early universe, Swinburne University of Technology   

    From astrobites: “Live fast, die young: quiescent galaxies in the early universe” 

    Astrobites bloc

    Astrobites

    Apr 18, 2017
    Christopher Lovell

    Title: A massive, quiescent galaxy at redshift of z=3.717
    Authors: Karl Glazebrook, Corentin Schreiber, Ivo Labbé, Themiya Nanayakkara, Glenn G. Kacprzak, Pascal A. Oesch, Casey Papovich, Lee R Spitler, Caroline M. S. Straatman, Kim-Vy H. Tran, Tiantian Yuan
    First author’s institution: Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia

    Status: Submitted for publication in NATURE, Open Access

    Galaxies in the early universe tend to be young and carefree. They have plenty of gas, and set about vigorously forming lots of stars. As a galaxy gets older though, it starts to run out of gas and becomes quiescent, no longer forming stars (see these bites for more details on quiescent galaxies). Theorists predict that it takes at least a few gigayears to deplete the gas, and this can be sped up by mergers and interactions with other galaxies. So, the further away we look (which corresponds to looking further back in time) the fewer quiescent galaxies we expect to see.

    Today’s paper is about the snappily named ZF-COSMOS-20115, a quiescent galaxy at the unusually high redshift of 3.7, around one and a half billion years after the big bang.

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    RARE FIND Galaxy ZF-COSMOS-20115, illustrated here, may be an oddity in the early universe. It formed stars rapidly, but then suddenly shut off, becoming red and dead by the time the universe was only 1.65 billion years old. Leonard Doublet/Swinburne University of Technology

    It has a mass equivalent to 170 billion suns, making it one of the most massive galaxies at this point in the universe’s history (much bigger than other similarly quiescent galaxies at this time), but it’s also very compact, less than a kiloparsec across (in comparison, our own Milky Way is ~ 50 kiloparsecs across). How did ZF-COSMOS-20115 become quiescent so quickly after forming, and is it a challenge to our current understanding of galaxy evolution?

    1
    Figure 1: Images of ZF-COSMOS-20115 with the Hubble Space Telescope (left panel), and from ground based telescopes (right panels). The left and top right images show the near-infrared, and the galaxy is clearly visible. In bottom right panel, showing visible light, the galaxy is undetected.

    In order for this galaxy to have formed so many stars and then become quiescent it must have had an enormous burst of star formation very early in its history. The authors speculate that such a burst could have been caused by a major merger between two similarly sized galaxies. Such a violent collision would have caused a huge amount of star formation in a relatively short period of time, sufficient to use up the gas reserves of both galaxies and prevent any further star formation after the merger.

    The authors argue that many current galaxy evolution models struggle to explain ZF-COSMOS-20115 – they contain galaxies of the right mass, but are still forming lots of stars. However, since this pre-print was released many of the theorists working on such models have retorted with evidence that they can produce analogues of ZF-COSMOS-20115 (see here and here). Whether these model analogues are really capturing the true nature of this galaxy or not is still up for debate. Future observations of more quiescent galaxies in the early universe will help theorists build a better picture of these young galaxies, tragically quiescent before their time…

    See the full article here .

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    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 12:25 pm on April 17, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Reionization of Dwarfs in the Local Group   

    From astrobites: “Reionization of Dwarfs in the Local Group” 

    Astrobites bloc

    Astrobites

    Title: Reionization of the Milky Way, M31, and their satellites I: Reionization History and Star Formation
    Authors: K. L. Dixon, I. T. Iliev, S. Gottlober, G. Yepes, A. Knebe, N. Libeskind, Y. Hoffman
    First Author’s Institution: Astronomy Centre, Department of Physics & Astronomy, University of Sussex, Falmer, Brighton, UK
    1
    Status: Submitted to MNRAS, open access

    There was a time in the universe when there were no stars. It was only after a long, dark 100 million years or so that the first stars—giant, blindingly bright monoliths a species apart from today’s stars—blinked on. As these stars blazed to life, they unleashed vast amounts of energetic ultraviolet (UV) photons. The universe at that time was filled mostly with cold hydrogen gas—cold enough to be in neutral form, single atoms of hydrogen. The UV photons unleashed by the stars changed this—they heated up the hydrogen gas and broke the atoms up into bare protons and electrons. This process was so efficient that the entire universe was reionized.

    Reionization era and first stars, Caltech

    In what seems like a cosmic fluke, it appears that much of the UV photons may have come from small galaxies—those at least a thousand times less massive than the Milky Way, which we call “dwarf” galaxies. For despite their diminutive size, there were legions of them—easily a hundred, if not a thousand dwarfs for each Milky Way-mass galaxy. Thus although each dwarf produced far less UV than a larger, Milky Way type galaxy, their sheer numbers caused them to produce more UV combined than larger galaxies. Their prodigious UV output also ushered in their fall—reionization seems to have slowed or even halted star formation in dwarfs, possibly by heating up or even evaporating all the star-forming, cold gas within them.

    Our own galactic neighborhood, also called the Local Group—the corner of the universe with our home galaxy, the Milky Way and our sister the Andromeda Galaxy (also known more arcanely as as Messier31), hosts dozens of dwarf galaxies that should have undergone precisely this.

    Local Group. Andrew Z. Colvin 3 March 2011

    Messier 31 Andromeda Galaxy NASA/ESA Hubble

    Thus the Local Group is a potentially illuminating place to look for clues as to how reionization affected star formation in dwarfs. But it’s not entirely clear what to search for. Would reionization produce dwarfs that completely stopped forming stars during reionization? Or those that only gradually kept forming stars? Should we expect stars in only the most massive dwarfs? We’re not sure.

    To rescue us from this quandary are the authors of today’s paper. They took a simulation of the Local Group and an adjacent group of galaxies, the nearby Virgo Cluster, and modeled how reionization may have taken place.

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    The Virgo Supercluster in supergalactic coordinates. Wikipedia

    To take into account the fact that we don’t know how much stars and UV the dwarf galaxies produced, they ran their simulation four times, identical except for how much ionizing UV the dwarfs contributed. In the first, the dwarfs contributed nothing at all—only the largest galaxies produced stars and reionizing UV. In the second, dwarfs contributed only if they were fairly isolated, far from larger galaxies. In the third, the dwarfs also contributed when close to a larger galaxy, although with reduced output. The last simulation was identical to the third, except that the amount a dwarf’s UV output was reduced depended on its mass—more massive dwarfs were more efficient at generating UV than lower mass dwarfs.

    What did they find? How early you reionized depends on how dense your corner of the universe is—Virgo, the most dense region between itself, the Milky Way, and M31, reionized first, and then was followed by Messier 31, and then the the Milky Way, the least dense of them all. In addition, in all cases in which dwarfs contributed, Virgo, M31, and the Milky Way appeared to reionize mostly “inside out”—the galaxies within reionized themselves, and then their surroundings. Once reionization got underway in the Local Group, it quickly progressed (faster than the rest of the universe), but slowed when about half the gas was ionized. The rest of the gas was reionized externally: it picked up again when an ionizing front from outside the Group swept through (see Figure 1).

    4
    Figure 1. Ionization of our galactic neighborhood. The time axis, measured in redshift, is backward (i.e. time starts at high redshift, on the right, and then progresses to lower redshift, towards the left). Each panel shows one of the four simulations run in this paper (see preceding paragraph). On the lower panels is the fraction of the hydrogen gas ionized over time for each sim. You can see the Virgo Cluster, shown in purple, reionized first, followed by the Local Group, shown in blue. The average fraction in the entire universe is shown in black. The top panels show how quickly Virgo or the Local Group reionized compared to the universe. When they first reionize, the rates are negative—they reionize faster than the universe, likely because the galaxies within Virgo and the Local Group are ionizing themselves quickly. The rate eventually becomes positive—reionization slows down. In the Local Group, it then picks back up at the end of reionization (at z~6), as outside sources ionize the rest of the gas. Figure taken from today’s paper.

    For the dwarfs, the authors found that the most massive ones generally reionized first, and that most satellites in the Local Group were ionized around the same time. But it appears difficult to determine when a Milky Way dwarf was ionized based on where it is today. In general, those that are closer to the Milky Way today appear to have formed most of their stars before reionization. Only about 20% of the dwarfs were able to form stars before reionization was complete—given that we see many low mass dwarfs that appear to have formed stars after reionization, this rules out scenarios in which dwarfs did not produce UV (and thus stars) after they came in close proximity to the Milky Way.

    It’s clear that reionization was complex, that and a particular dwarf’s ability to form stars is dependent on details about its environment during reionization. Simulations that model this era of cosmic history in more detail (such as reionization due to supernovae) will help us better understand the role that dwarfs played in reionization, and how they were affected by it.

    See the full article here .

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