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  • richardmitnick 1:24 pm on March 16, 2017 Permalink | Reply
    Tags: , , , , Dwarf galaxies, , , When the Neighborhood Dwarf Galaxies were Kids   

    From astrobites: “When the Neighborhood Dwarf Galaxies were Kids’ 

    Astrobites bloc

    Astrobites

    Title: Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era
    Authors: D. R. Weisz, M. Boylan-Kolchin
    First Author’s Institution: Department of Astronomy, University of California Berkeley, Berkeley, CA
    1
    No image credit
    Status: Submitted to MNRAS, [open access]

    The heavens bespeak a dark and quiet night, glinting here and there with distant stars and yet more distant galaxies. But in ages past, long before the birth of our stalwart Sun, before even the supernovae that spewed the calcium in our bones and the iron in our blood into the gas that formed the Sun and the Solar System, there was darkness. The cosmic dark ages reigned for nearly a million years before the first stars blinked blearily on.

    Then suddenly there came an age of light. We’re not entirely sure what exactly lit up the universe, but among the suspects are the first galaxies. Once practically invisible, they were lit aflame as the first stars began to burn hot and bright within them. They generated copious amounts of ultra-violet (UV) light, energetic enough to ionize the hydrogen in the universe. So much UV flux was generated that nearly all the hydrogen in the universe was ionized, leaving the universe clear and transparent and allowing us the majestic views of faraway galaxies that we take for granted today.

    2
    Figure 1. The role of small galaxies like the Milky Way’s newly discovered ultra-faint galaxies could have had in reionizing the universe. The top panel shows the number density of galaxies as a function of how bright they were in the ultra-violet (MUV), what we call the Salpeter function. The colors denote how the numbers changed with time; purple (z ~ 8) denotes when reionization occurred, and the lighter colors denote the subsequent evolution after reionization. Far more faint galaxies (less negative MUV) exist compared to brighter galaxies. Together, they produced most of the UV flux during reionization, as shown in the middle and bottom panels: the middle panel shows the density of UV photons (which is clearly highest at the faint end), and the bottom panel shows the cumulative fraction of the flux that galaxies brighter than a given MUV were generating. As much as 50-80% of the UV flux that reionized the universe may have come from galaxies fainter than MUV ~ -10! Figure taken from today’s paper.

    The authors of today’s paper investigate what role the newly discovered ultra-faint dwarf galaxies orbiting our Milky Way could have had during this epoch of reionization. For it turns out that our most careful galaxy counts—which we’ve codified into what we now call the Schechter function (see Figure 1)—tell us that the universe swarms with dwarf galaxies, which are at least a thousand times less massive than the more familiar grand spirals such as the Milky Way. The Milky Way itself is thought to be surrounded by several hundreds, if not thousands, of such galaxies. Dwarf galaxies are so numerous that together, they may have been able to provide much of the UV photons required. It was an age in which the smallest galaxies ruled the ultra-violet skies.


    Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array


    U Toronta Dragon Fly Telescope Array

    We don’t know for sure if dwarf galaxies can solve the mystery of reionization. The problem is that it’s extremely difficult to peer into the universe’s distant past, and literally impossible to observe the faint dwarfs that existed then. Our best observations hint that there were many more dwarfs in the past than in the present, before they were torn apart and cannibalized by larger galaxies.

    To work around these uncertainties, the authors did something simple. They worked out when the stars in the Milky Way’s ultra-faint dwarfs formed to determine how much UV light they’d give off during reionization. They then asked: If the Schechter function was valid for dwarfs as dim as the Milky Way’s faintest dwarfs used to be, how much UV light would they have produced? They found that dwarfs could have generated as much as 50-80% of the UV photons needed to reionize the universe.

    There’s hints, however, that such a simple extrapolation of the Schechter function to ultra-faint galaxies overestimates the number of bright UV-emitting dwarfs. The Schechter function predicts that we should see as many as ten times as many bright dwarfs around the Milky Way than we actually do. And it’s becoming clear that the smallest galaxies have trouble producing UV-generating stars. This would cause the Schechter function to “turn over” (see Figure 2) or predict fewer bright dwarfs (and hordes of small, dark galaxies). The authors show that the estimated reduction in bright dwarfs seen in simulations lowers the UV flux we should expect from small galaxies to about 10%.

    Although we haven’t yet gotten to the bottom of how large a role dwarf galaxies played in the ionization of the universe, it’s clear that the yet unobservable number of UV-bright dwarfs matters greatly in understanding how the history of the universe unfolded. The upcoming James Webb Space Telescope has the ability to detect galaxies from the epoch of reionization that are almost 100x fainter—still far short of the 10,000x increase in sensitivity we need to see the faint UV galaxies that preoccupied today’s authors. It’ll be a big step forward, but we’ll still have to hunt for other clues as to the true numbers of dwarf galaxies during reionization.

    3
    Figure 2. How a reduction in the number of ultra-faint dwarf galaxies can reduce their contribution to the reionization of the universe. The top panel shows different guesses as to the number density of galaxies of a given UV brightness. In black is the traditional Salpeter function, which predicts many faint UV galaxies, while the orange and purple curves are based on simulations that show a “turn-over” or reduction in the number of faint UV galaxies. The bottom panel shows that if there are just a few UV-faint galaxies, then they contribute only ~10% of the UV flux that reionized the universe. Figure taken from today’s paper.

    See the full article here .

    Please help promote STEM in your local schools.

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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 8:38 pm on February 15, 2017 Permalink | Reply
    Tags: , , ‘Hierarchical’ assembly, , Dark matter haloes, Dwarf galaxies, Gravitationally bound structures, TiNy Titans survey (TNT)   

    From astrobites: “Honey, I found Isolated Dwarfs!” 

    Astrobites bloc

    Astrobites

    Feb 15, 2017
    Bhawna Motwani

    Title: Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups
    Authors: S. Stierwalt, S. E. Liss et al.
    First Author’s Institution: National Radio Astronomy Observatory (NRAO) & University of Virginia, Charlottesville VA, USA
    NRAO Small
    UVA bloc
    Status: Published in Nature Astronomy, open access

    The current favourite model for the evolution of the universe, the Lambda Cold Dark Matter (LCDM) model, supports growth of cosmological structure via consolidation of smaller units.

    1
    Lambda-Cold Dark Matter Photo by DVDjHex | Photobucket

    Widely referred to as ‘hierarchical’ assembly, this prescription posits that dark matter haloes as small as the size of our solar system act as the first seedlings that gradually grow up to be galaxies, galaxy groups and galaxy clusters. As a natural consequence of this picture, cosmological simulations predict a huge extant population of satellite structures surrounding the present-day structure at all scales that survived during the latter’s build-up process.

    So, where are these satellites; have we seen them?

    The answer, as it turns out, is yes and no. Observations have clearly elucidated that big galaxies such as our own Milky Way have several satellite- (or ‘dwarf’) galaxies surrounding them, as well as remnants of their destroyed building blocks in the form of stellar steams. On the other hand, despite predictions from theory and simulations, no satellites have been observed around the dwarfs themselves, nor have any dwarf galaxies been observed far away from big galaxies. Naturally, this has posed to be a discouraging evidence against the hierarchical buildup at small scales so far.

    The Respite:

    At the beginning of this year, Sabrina Stierwalt and her collaborators brought water to the thirsty by publishing the long-sought evidence of hierarchical structure formation at the low mass scale. In their paper, the authors reported direct observations of seven isolated, compact galaxy groups comprised solely of dwarf galaxies (see Figure 1).

    2
    Figure 1. A three-color composite image of one of the observed groups, where red objects depict the individual member dwarf galaxies.

    The discovery of these groups was made during a visual inspection of the most isolated dwarf galaxy pairs in the TiNy Titans survey (TNT), a multi-wavelength observational campaign that aims to investigate the effect of dwarf–dwarf interactions on the evolution of low-mass galaxies.

    3
    Broadband optical (SDSS ugriz) images of 7 members of our TNT Pilot Survey. These isolated pairs illustrate a plausible dwarf-dwarf merger sequence; they are organized by increasing projected separation rp = 0.85, 5.35, 10.01, 15.20, 23.43, 45.10, 49.61 kpc. Bright blue knots reveal sites of ongoing SF that may have been enhanced due to a recent interaction and many exhibit strong tidal features.

    Even though they say seeing is believing, in astronomy, seeing something alone is rarely enough. In order to establish the identity of the objects the authors had seen as dwarf galaxy groups, they performed follow-up spectroscopy to confirm the association of the candidate dwarf galaxies with the visual groups in their images. Using the information about the groups’ projected sizes and velocity dispersions (see Figure 2), combined with the knowledge of typical dark matter content for dwarf galaxies, the authors performed dynamical mass calculations, the results of which imply that the observed associations are likely gravitationally bound structures.

    4
    Figure 2. Projected radial distance from the centroid of the group vs. difference in group member line-of-sight velocity from the group mean. The seven different symbols represent dwarfs belonging to the seven groups detected by the authors.

    This isn’t the first time that associations of dwarf galaxies have come into the limelight. Previously made observations of the Milky Way dwarfs and their apparent proximity to the orbital plane of the the Large Magellanic Cloud have been argued to suggest that those dwarfs could be the result of a tidal breakup of the Magellanic group, of which the Magellanic Clouds were the largest (and brightest) members.

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

    Large Magellanic Cloud. Adrian Pingstone  December 2003
    Large Magellanic Cloud. Adrian Pingstone December 2003

    Nonetheless, what makes the dwarf groups described by today’s authors in their paper truly unique relative to any previously known associations is their virtue of being highly compact and isolated. Being about an order of magnitude less extended than previous groups, and more than five million light years away from any massive neighbor, the TNT groups have the potential to serve as ideal labs for the study of structure build-up at small scales, unaffected by sensitive environmental effects such as ram pressure or tidal stripping that can otherwise erase the dynamical signatures of historically existing coherent structure.

    The discovery of TNT dwarf groups provides a promising opportunity for the study of hierarchical assembly at low mass scales. However, mis-judgement of information sprouting up due to the completeness effect, and a bias towards detection of bright galaxies are possible in this study. Given that the brightest members of the reported groups are rather large, this study keeps the story of hierarchical formation at typical dwarf and satellite galaxy masses (i.e., very low mass-scales) yet a mystery. Future observations of even fainter galaxies and substructure in these groups will boost the census of dwarf galaxies to a statistically significant level, providing stronger grounds to tune our understanding of how structure forms in this elusive universe of ours.

    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 9:29 am on January 24, 2017 Permalink | Reply
    Tags: , , , Dwarf galaxies   

    From COSMOS: “Seven elusive dwarf galaxy groups revealed” 

    Cosmos Magazine bloc

    COSMOS

    24 January 2017
    No writer credit found

    1
    Four dwarf galaxies identified by astronomers. These tiny galaxies can offer insight into the formation of larger ones, such as the Milky Way.
    Kelsey E Johnson, Sandra E Liss and Sabrina Stierwalt.

    The discovery forms an early piece of the galactic evolution puzzle.

    A piece of the galactic growth chart has been revealed with seven gangs of tiny galaxies, long-sought by astronomers, confirmed in Nature Astronomy.

    The finds provide insights into how mid-sized galaxies, such as our own Milky Way, formed.

    Astronomers think most medium-to-large galaxies grew through collisions. You can see evidence for such mergers – streams of stars and gas can be flung out as two galaxies combine.

    The Milky Way and our nearest major galactic neighbour Andromeda are on a collision course, tipped to combine into a larger galaxy in around four billion years.

    Of course, that’s a long wait. So researchers find and examine groups of dwarf galaxies, 10 to 1,000 times smaller than the Milky Way, to see if they might show signs of such mergers.

    The problem is dwarf galaxies are hard to find, let alone groups of them. Given the universe is 13.8 billion years old, it’s harder still to find galactic groups out on their own in space consisting only of dwarfs.

    Systems previously identified were quite close to a large galaxy, or the galaxies in the group were very far from each other – conditions that could affect their behaviour.

    So to hunt down tightly bound dwarf galaxy congregations that were far enough from massive galaxies, Sabrina Stierwalt from the National Radio Astronomy Observatory in the US and colleagues searched the Sloan Digital Sky Survey for dwarf galaxy pairs. They turned up 60 candidates.

    Confirmation came from observations from telescopes such as the 3.5-metre telescope at Apache Point Observatory in the US and Magellan Telescope in Chile. And given time, it’s thought they’ll merge into intermediate-mass galaxies.

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    Carnegie 6.5 meter Magellan  Baade and Clay Telescopes located at Carnegie’s Las Campanas Observatory, Chile.
    Carnegie 6.5 meter Magellan Baade and Clay Telescopes located at Carnegie’s Las Campanas Observatory, Chile.

    They even found one dwarf galaxy, dubbed DDO68, which appeared to be the product of a collision of two even tinier galaxies. It too had star streams indicating a merger.

    1
    DDO68. https://www.sao.ru/Doc-en/SciNews/LBV-DDO68/

    This example, the researchers write, will be “fertile ground” for future telescopes and deeper surveys such as the planned Large Synoptic Survey Telescope.

    LSST
    LSST/Camera, built at SLAC
    LSST/Camera, built at SLAC
    LSST Interior
    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.
    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.

    See the full article here .

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  • richardmitnick 9:50 am on May 1, 2016 Permalink | Reply
    Tags: , , Crater 2 dwarf galaxy, Dwarf galaxies,   

    From Sky and Telescope: “Milky Way’s New Neighbor: A Giant Dwarf” 

    SKY&Telescope bloc

    Sky & Telescope

    April 27, 2016
    John Bochanski

    Astronomers have discovered a “feeble giant”: one of the largest dwarf galaxies ever seen near the Milky Way.

    Ever since astronomers discovered our universe’s accelerating expansion, tension has rippled between theory and observations, especially in studies of our galaxy’s neighborhood.

    The standard model of cosmology, which suggests that dark energy and “cold” dark matter govern the universe’s evolution, predicts many more small galaxies near the Milky Way than what we’ve observed so far. Dwarfs should be the building blocks of larger galaxies like our own, so the lack has puzzled astronomers — are they not there, or are we just not seeing them?

    Observations have closed in on theory in recent years with the advent of large surveys such as the Sloan Digital Sky Survey and the Dark Energy Survey, where observers have begun to identify hard-to-find dwarf galaxies. Dozens of dwarfs have been spotted over the last 15 years. But theory suggests perhaps even hundreds more have yet to be discovered.

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    Dark Energy Icon
    Dark Energy Camera,  built at FNAL
    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, ChileCTIO Victor M Blanco 4m Telescope interior
    DECam, built at FNAL; the NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile

    Now, the list of known dwarfs has just added one of its largest members: Crater 2 [no image available]. You’d think large dwarfs would be easy to find, but this one’s stars are spread out and easily entangled with the stars of the Milky Way. It took a sensitive survey to pick out the small galaxy hidden behind the galaxy’s stars.

    A New Dwarf Galaxy

    Gabriel Torrealba (University of Cambridge, UK) led a team that discovered the Crater 2 dwarf galaxy in survey data collected at the Very Large Telescope in Chile.

    ESO/VLT at Cerro Paranal, Chile
    ESO/VLT at Cerro Paranal, Chile

    The team used specialized software to spot over-crowding among stars, searching for dim stellar clumps. But identifying a clump isn’t enough. Only Crater 2 contained red giant stars and horizontal branch stars — both old, evolved stars that mark an ancient stellar population separate from the youthful Milky Way disk.

    Torrealba and colleagues estimate that Crater 2 lies 391,000 light-years from Earth. That makes it one of the most distant dwarf galaxies known. It’s also one of the largest: at 6,500 light-years across, it comes in fourth among our galaxy’s neighbors, after the Large and Small Magellanic Clouds, and the torn-apart Sagittarius dwarf galaxy. Moreover, it’s incredibly diffuse, its stars spread out over several square degrees. So despite its size, Crater 2 is much fainter than those Milky Way companions, nearly 100 times fainter than Sagittarius and almost 10,000 times fainter than the LMC.

    Dwarf Galaxy Groups

    The discovery of Crater 2 may help unlock an ongoing puzzle in the Milky Way’s evolution. As astronomers began to discover dwarf galaxies en masse in large sky surveys, it soon became clear that some dwarfs cluster in their orbits. Crater 2 is no exception: the team estimated that the dwarf’s orbit lines up with those of the Crater globular cluster, as well as the Leo IV, Leo V and Leo II dwarf galaxies.

    Dwarf Galaxies with Messier 101  Allison Merritt  Dragonfly Telephoto Array
    Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array

    U Toronto Dunlap Dragonfly telescope Array
    U Toronto Dunlap Dragonfly telescope Array

    While not a definitive association, similar orbits suggest that these objects might form a group that fell together into our galaxy’s gravitational well. Astronomers have recently found similar groups near the Large Magellanic Cloud, suggesting that our galaxy’s halo might have formed through many such group captures.

    Large Magellanic Cloud. Adrian Pingstone  December 2003
    Large Magellanic Cloud. Adrian Pingstone December 2003

    As sky surveys continue to enable discoveries of dwarf galaxies such as Crater 2, the gap between theory and observations continues to narrow, clarifying our understanding of the Milky Way’s evolution. The future is bright for the study of these dim galaxies, thanks to surveys such as the Large Synoptic Sky Survey (LSST) on the horizon. LSST will push to even fainter magnitudes and may finally resolve the discrepancy between theory and observation.

    LSST/Camera, built at SLAC
    LSST Interior
    LSST telescope, currently under construction at Cerro Pachón Chile
    LSST/Camera, built at SLAC; LSST telescope, currently under construction at Cerro Pachón Chile

    Reference:
    G. Torrealba et al. The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater. Accepted for publication in Monthly Notices of the Royal Astronomical Society.

    See the full article here .

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    Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

    Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

    Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

    “Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

     
  • richardmitnick 1:38 pm on April 15, 2016 Permalink | Reply
    Tags: , , Dwarf galaxies,   

    From New Scientist: “Never-before-seen galaxy spotted orbiting the Milky Way” 

    NewScientist

    New Scientist

    14 April 2016
    Ken Croswell

    1
    The Milky Way is orbited by 49 other galaxies – that we know of. ESO

    The galaxy’s empire has a new colony. Astronomers have detected a dwarf galaxy orbiting the Milky Way whose span stretches farther than nearly all other Milky Way satellites. It may belong to a small group of galaxies that is falling into our own.

    Giant galaxies like the Milky Way grew large when smaller galaxies merged, according to simulations. The simulations also suggest that whole groups of galaxies can fall into a single giant at the same time. The best examples in our cosmic neighbourhood are the Large and Small Magellanic Clouds, the Milky Way’s two brightest satellites, which probably orbit each other.
    Orbiting galaxies

    About four dozen known galaxies orbit our own. The largest in terms of breadth is the Sagittarius dwarf, discovered in 1994 – but it’s big only because our galaxy’s gravity is ripping it apart. The next two largest are the Magellanic Clouds.

    Now, Gabriel Torrealba at the University of Cambridge and his colleagues* have found a new galaxy about 380,000 light years away in the constellation Crater. “It’s the fourth largest satellite of the Milky Way,” Torrealba says.

    Named the Crater 2 dwarf, the new galaxy is not apparent to human eyes, though individual stars within the galaxy are visible. The team were only able to find it this January by using a computer to look for over-densities of stars in data from images taken by a telescope in Chile [ESO VST].

    ESO VST telescope
    ESO VST Interior
    ESO VST telescope

    How do you measure a galaxy?

    Most galaxies don’t have defined edges, so astronomers sometimes express a galaxy’s size in terms of its “half-light diameter”, which encloses the brightest part of the galaxy and emits half of its light. The Crater 2 dwarf has a half-light diameter of 7000 light years – which, if we could see it, would look twice as big as the full moon.

    Josh Simon, an astronomer at the Carnegie Observatories in Pasadena, California, says the galaxy is notable because it is brighter than nearly all of the many galaxies found orbiting the Milky Way during the past decade. It emits 160,000 times more light than the sun.
    Ghostly appearance

    The galaxy eluded detection for so long because its stars are spread out from one another, giving it a ghostly appearance.

    Torrealba says it may not be alone. The Crater 2 dwarf is near four other new-found objects: the Crater globular star cluster as well as three dwarf galaxies in Leo. All may be part of a group that is just now falling into the Milky Way.

    Until now, though, the new galaxy has led a quiet life, never venturing near a giant galaxy. We know this because the galaxy is round. If it had encountered a giant, gravity would have bent the dwarf out of shape.

    Reference*: Monthly Notices of the Royal Astronomical Society, in press, and arxiv.org/abs/1601.07178

    Science paper:
    *The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater

    Science team:
    G. Torrealba1, S.E. Koposov1, V. Belokurov1 & M. Irwin1

    Affiliations:
    1Institute of Astronomy, Madingley Rd, Cambridge, CB3 0HA

    See the full article here .

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  • richardmitnick 5:56 pm on March 9, 2016 Permalink | Reply
    Tags: , , , Dwarf galaxies,   

    From UC Riverside: “Dark Matter Satellites Trigger Massive Birth of Stars” 

    UC Riverside bloc

    UC Riverside

    March 9, 2016
    Sean Nealon

    Dwarf Galaxies with Messier 101  Allison Merritt  Dragonfly Telephoto Array
    Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array

    U Toronto Dunlap Dragonfly telescope Array
    U Toronto Dunlap Dragonfly telescope Array

    One of the main predictions of the current model of the creation of structures in the universe, known at the Lambda Cold Dark Matter model, is that galaxies are embedded in very extended and massive halos of dark matter that are surrounded by many thousands of smaller sub-halos also made from dark matter.

    Around large galaxies, such as the Milky Way, these dark matter sub-halos are large enough to host enough gas and dust to form small galaxies on their own, and some of these galactic companions, known as satellite galaxies, can be observed. These satellite galaxies can orbit for billions of years around their host before a potential merger. Mergers cause the central galaxy to add large amount of gas and stars, triggering violent episodes of new star formation −known as starbursts− due to the excess gas brought in by the companion. The host’s shape or morphology can also be disturbed due to the gravitational interaction.

    Smaller halos form dwarf galaxies, which at the same time will be orbited by even smaller satellite sub-halos of dark matter which are now far too tiny to have gas or stars in them. These dark satellites therefore are invisible to telescopes, but readily appear in theoretical models run in computer simulations. A direct observation of their interaction with their host galaxies is required to prove their existence.

    Laura Sales, an assistant professor at the University of California, Riverside’s Department of Physics and Astronomy, collaborated with Tjitske Starkenburg and Amina Helmi, both of the Kapteyn Astronomical Institute in The Netherlands, to present a novel analysis of computer simulations, based on theoretical models, that study the interaction of a dwarf galaxy with a dark satellite.

    The findings were outlined in a just-published paper, Dark influences II: gas and star formation in minor mergers of dwarf galaxies with dark satellites, in the journal Astronomy & Astrophysics.

    The researchers found that during a dark satellite’s closest approach to a dwarf galaxy, through gravity it compresses the gas in the dwarf, triggering significant episodes of starbursts. These star forming episodes may last for several billions of years, depending on the mass, orbit and concentration of the dark satellite.

    This scenario predicts that many of the dwarf galaxies that we readily observe today should be forming stars at a higher rate than expected —or should be experiencing a starburst— which is exactly what telescope observations have found.

    Furthermore, similarly to mergers between more massive galaxies, the interaction between the dwarf galaxy and the dark satellite triggers morphological disturbances in the dwarf, which can completely change its structure from mainly disk-shaped to a spherical/elliptical system. This mechanism also offers an explanation to the origin of isolated spheroidal dwarf galaxies, a puzzle that has remained unsolved for several decades.

    See the full article here .

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    UC Riverside Campus

    The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

    Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

    We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

     
  • richardmitnick 3:42 pm on January 11, 2016 Permalink | Reply
    Tags: , , , Dwarf galaxies   

    From AAS NOVA: ” The Hunt for Dwarf Galaxies’ Ancestors” 

    AASNOVA

    American Astronomical Society

    11 January 2016
    Susanna Kohler

    Temp 1
    Hubble image of the Sagittarius Dwarf Galaxy in the local group.

    2
    Local Group. Andrew Z. Colvin

    Given how faint these galaxies are, what is the likelihood we’ll be able to find their distant ancestors? [NASA, ESA, and The Hubble Heritage Team (STScI/AURA)]

    Dwarf galaxies are typically very faint, and are therefore hard to find. Given that, what are our chances of finding their distant ancestors, located billions of light-years away? A recent study aims to find out.

    Ancient Counterparts

    Dwarf galaxies are a hot topic right now, especially as we discover more and more of them nearby. Besides being great places to investigate a variety of astrophysical processes, local group dwarf galaxies are also representative of the most common type of galaxy in the universe. For many of these dwarf galaxies, their low masses and typically old stellar populations suggest that most of their stars were formed early in the universe’s history, and further star formation was suppressed when the universe was reionized at redshifts of z ~ 6–10. If this is true, most dwarf galaxies are essentially fossils: they’ve evolved little since that point.

    To test this theory, we’d like to find counterparts to our local group dwarf galaxies at these higher redshifts of z = 6 or 7. But dwarf galaxies, since they don’t exhibit lots of active star formation, have very low surface brightnesses — making them very difficult to detect. What are the chances that current or future telescope sensitivities will allow us to detect these? That’s the question Anna Patej and Abraham Loeb, two theorists at Harvard University, have addressed in a recent study.

    Entering a New Regime

    Temp 2
    The surface brightness vs. size for 73 local dwarf galaxies scaled back to redshifts of z=6 (top) and z=7 (bottom). So far we’ve been able to observe high-redshift galaxies within the boxed region of the parameter space. JWST will open the shaded region of the parameter space, which includes some of the dwarf galaxies. [Patej & Loeb 2015]

    Starting from observational data for 87 Local-Group dwarf galaxies, Patej and Loeb used a stellar population synthesis code to evolve the galaxies backward in time to redshifts of z = 6 and 7. Next, they narrowed this sample to only those dwarfs for which most star formation had already occurred by this time.

    Finally, the authors compared the properties of these 73 scaled-back dwarfs to those of high-redshift galaxies that we have already detected with the Hubble and Spitzer Space Telescopes, as well as to the detection limits of the upcoming James Webb Space Telescope (JWST) mission launching in 2018.

    NASA Hubble Telescope
    NASA/ESA Hubble

    NASA Spitzer Telescope
    NASA/Spitzer

    NASA Webb Telescope
    NASA/Webb

    Patej and Loeb find that, when scaled back to redshifts of z = 6 or 7, the dwarf galaxies would be too faint to detect with current telescopes — despite being roughly the same size as high-redshift galaxies we’ve already detected. But the capabilities of JWST will push into this regime: according to Patej and Loeb’s calculations, JWST would be able to detect 13 of the 73 galaxies in the sample at a redshift of z = 6, and 9/73 at a redshift of z = 7.

    Furthermore, the fraction of detectable galaxies would increase if these ancient dwarfs contained large numbers of Population-III-like, massive, bright stars. But even without such a boost, the hunt for the ancestors of local dwarf galaxies appears to be well within JWST’s capabilities!

    Citation

    Anna Patej and Abraham Loeb 2015 ApJ 815 L28. doi:10.1088/2041-8205/815/2/L28

    See the full article here .

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  • richardmitnick 2:30 pm on January 5, 2016 Permalink | Reply
    Tags: , , Dwarf galaxies,   

    From Symmetry: “The booming science of dwarf galaxies” 

    Symmetry

    01/05/16
    Manuel Gnida

    1
    Dragonfly Telephoto Array Discovers Seven Dwarf Galaxies around Messier 101

    U Toronto Dunlap Dragonfly telescope Array
    U Toronto Dunlap Institute Dragonfly Array

    2
    Messier 101 (Pinwheel Galaxy) This image is from NASA/ESA Hubble It is presented here so that the viewer can approximate the locations of the dwarf galaxies. Dwarf galaxies are associated with most major galaxies.

    NASA Hubble Telescope
    NASA/ESA Hubble

    A recent uptick in the discovery of the smallest, oldest galaxies benefits studies of dark matter, galaxy formation and the evolution of the universe.

    Galaxies are commonly perceived as gigantic spirals full of billions to trillions of stars. Yet some galaxies, called dwarf galaxies, can harbor just a few hundred suns.

    The recent discovery of 20 new potential dwarf galaxies fueled a boom in the science of these faint objects, which are valuable tools to study dark matter, galaxy formation and cosmic history.

    Ten years ago, only about a dozen dwarf galaxies were known. This number quickly doubled after the Sloan Digital Sky Survey [SDSS] began its second phase of operation in 2005. SDSS-II took better than ever images of the sky, and researchers started using computer programs to identify dwarf galaxies in them.

    SDSS Telescope
    SDSS teleecope at Apache Point, NM, USA

    The number of potential dwarf galaxies has spiked of late, largely due to results from the first two years of the new Dark Energy Survey, which can see objects 10 times as faint.

    DECam
    CTIO Victor M Blanco 4m Telescope
    DECam, built at FNAL, and the CTIO Victor M Blanco telescope in Chile in which it is housed.

    The total number of known dwarf galaxy candidates orbiting our Milky Way—not all of them have been confirmed as galaxies yet—has now reached about 50.

    “The precise number is being updated on almost a weekly basis during recent months,” says Keith Bechtol of the University of Wisconsin, Madison, one of the lead authors of two DES papers, published in March and August, announcing the discoveries of potential satellite galaxies. “These are truly exciting times for this type of research.”

    Dim lights for dark matter research

    In general, the term “dwarf galaxy” refers to a galaxy that is smaller than a tenth of the size of our Milky Way, which is made of 100 billion stars. So not all dwarf galaxies are truly dwarfish. In fact, two of these objects in the southern night sky, called the Magellanic Clouds, are so large that they are visible to the naked eye.

    4
    Large Magellanic Cloud. Adrian Pingstone in December 2003

    6
    Small Magellanic Cloud (SMC) via ESO/Digitized Sky Survey 2

    However, researchers are particularly interested in the faintest dwarf galaxies. They make excellent laboratories in which to study dark matter—the invisible form of matter that is five times more prevalent than its visible counterpart but whose nature remains a mystery.

    Scientists’ best guess is that it’s made of fundamental particles, with hypothetical weakly interacting massive particles, or WIMPs, as the top contenders. Researchers think WIMPs could produce gamma rays as they decay or annihilate each other in space. They’re searching for this radiation with sensitive gamma-ray telescopes.

    Ultra-faint dwarf galaxies orbiting the Milky Way are ideal targets for this search for two reasons. First, because they have high ratios of dark matter to regular matter and are relatively close to us, they could produce detectable dark matter signals.

    “The motions of stars in ultra-faint galaxies are so fast that they are best explained if there is 100 to 1000 times more dark matter than the masses of all the stars taken together,” Bechtol says.

    Second, these galaxies are the oldest known galaxies. Their busiest days are in the past; most formed their stars more than 10 billion years ago. This, together with the fact that they have few stars and little gas, makes them very “clean” objects for the dark matter search.

    By contrast, in the also dark-matter-rich center of the much younger Milky Way, stars are still forming and many other astrophysical processes are producing gamma-ray signals that could obscure signs of dark matter.

    “If we ever saw gamma rays coming from these ultra-faint galaxies, it would be a smoking gun for dark matter,” says researcher Andrea Albert of the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and the SLAC National Accelerator Laboratory. She is involved in the dark matter analysis of the recently discovered DES dwarf galaxy candidates with the Fermi Gamma-ray Space Telescope.

    No convincing sign of WIMPs [Weakly interacting massive particles, being hunted as a possible constituent of dark matter] has yet been found coming from dwarf galaxies, including the most recent DES candidate dwarf galaxies, as preliminary results presented at the 2015 Topics in Astroparticle and Underground Physics conference in Torino, Italy, suggest.

    But even the absence of a signal is progress because it sets limits on what dark matter can and cannot be.

    Excavation tools for astrophysical archeology

    Researchers also study dwarf galaxies because they hope to learn about the history of our cosmic neighborhood and the formation of galaxies like our own.

    Current models suggest that galaxies don’t start out as enormous objects with a gazillion of stars, but rather as small structures that merge with others into larger ones. Dwarf galaxies are at the bottom of this hierarchy and are believed to be the building blocks of larger galaxies.

    “The way we see the Milky Way and its satellites today is only a snapshot in time,” says astronomer Marla Geha of Yale University. “Our own galaxy is a merger of smaller galaxies, and it’s still merging.”

    In other words, the Milky Way itself may have started out as a dwarf galaxy with only a few hundred to thousand stars. Today, it is a large galaxy, and simulations suggest that in 4 to 5 billion years the Milky Way will merge with the Andromeda Galaxy, the nearest major galaxy in our cosmic neighborhood.

    But dwarf galaxies can give us insight into more than just our local neighborhood. By better understanding dwarf galaxies, researchers can also study the evolution of the whole universe.

    Since dark matter is abundant and interacts gravitationally with itself and regular matter, it has influenced the cosmos and its structures ever since the big bang. In fact, we know today that galaxies are embedded in clumps, or halos, of dark matter that have formed in the expanding universe. These halos, in turn, are surrounded by smaller halos that harbor dwarf satellite galaxies.

    “The discovery of an increasing number of dwarf galaxies is exciting because our cosmological theories predict that the Milky Way has a few hundred of them,” Geha says. “If we don’t find enough satellites, we will need to adjust our models. However, considering that we haven’t surveyed the entire sky yet and haven’t been looking deeply enough, we’re mostly on track for our predictions.”

    Some of the dwarf galaxy candidates discovered by DES earlier this year could potentially orbit the Magellanic Clouds, the largest satellites of the Milky Way. If confirmed, the result would be quite fascinating, says KIPAC researcher Risa Wechsler.

    “Satellites of satellites are predicted by our models of dark matter,” she says. “Either we’re seeing these types of systems for the first time, or there is something we don’t understand about how these satellite galaxies are distributed in the sky.”
    A better and better view

    So far, studies of dwarf galaxies have largely been restricted to satellites of our Milky Way, and researchers believe that much could be learned from studying more distant ones.

    “One question we would like to answer is why the faintest dwarf galaxies are so extreme in size, age and dark matter content,” Geha says. “Is this because the ones we can observe are affected by their proximity to the Milky Way, or are these properties common to all dwarf galaxies in the universe?”

    For the ultimate test, researchers want to be able to detect even fainter objects and look farther into space than they can with DES. They’ll be able to do so once the Large Synoptic Survey Telescope will come online in 2022. The telescope’s 3.2-gigapixel camera will produce the deepest views of the night sky ever observed.

    And if that’s not enough, NASA is planning a space mission called the Wide-Field Infrared Survey Telescope, which could spot ultra-faint dwarf galaxies that evade even LSST’s watchful eye.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 3:44 pm on November 24, 2015 Permalink | Reply
    Tags: , , Dwarf galaxies,   

    From NOAO: “Oodles of Faint Dwarf Galaxies in Fornax Shed Light on a Cosmological Mystery” 

    NOAO Banner

    November 23, 2015
    Dr. Joan Najita
    National Optical Astronomy Observatory
    950 N Cherry Ave
    Tucson AZ 85719 USA
    +1 520-318-8416
    E-mail: najita@noao.edu

    1
    Image of the inner 3 square degrees of the NGFS survey footprint compared with the size of the Moon. Low surface brightness dwarf galaxies are marked by red circles. Gray circles indicate previously known dwarf galaxies. The dwarf galaxies, which vastly outnumber the bright galaxies, may be the “missing satellites” predicted by cosmological simulations.

    An astonishing number of faint low surface brightness dwarf galaxies recently discovered in the Fornax cluster of galaxies may help to solve the long-standing cosmological mystery of “The Missing Satellites”. The discovery, made by an international team of astronomers led by Roberto Muñoz and Thomas Puzia of Pontificia Universidad Católica de Chile, was carried out using the Dark Energy Camera (DECam) on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO). CTIO is operated by the National Optical Astronomy Observatory (NOAO).

    DECam
    CTIO Victor M Blanco 4m Telescope
    DECam (built at FNAL) and the CTIO Victor M Blanco telescope in Chile in which it is housed.

    Computer simulations of the evolution of the matter distribution in the Universe predict that dwarf galaxies should vastly outnumber galaxies like the Milky Way, with hundreds of low mass dwarf galaxies predicted for every Milky Way-like galaxy. The apparent shortage of dwarf galaxies relative to these predictions, “the missing satellites problem,” could imply that the cosmological simulations are wrong or that the predicted dwarf galaxies have simply not yet been discovered. The discovery of numerous faint dwarf galaxies in Fornax suggests that the “missing satellites” are now being found.

    The discovery, recently published in the Astrophysical Journal, comes as one of the first results from the Next Generation Fornax Survey (NGFS), a study of the central 30 square degree region of the Fornax galaxy cluster using optical imaging with DECam and near-infrared imaging with ESO’s VISTA/VIRCam. The Fornax cluster, located at a distance of 62 million light-years, is the second richest galaxy cluster within 100 million light-years after the much richer Virgo cluster.

    The deep, high-quality images of the Fornax cluster core obtained with DECam were critical to the recovery of the missing dwarf galaxies. “With the combination of DECam’s huge field of view (3 square degrees) and our novel observing strategy and data reduction algorithms, we were able to detect extremely diffuse low-surface brightness galaxies,” explained Roberto Muñoz, the lead author of the study.

    Because the low surface brightness dwarf galaxies are extremely diffuse, stargazers residing in one of these galaxies would see a night sky very different from that seen from Earth. The stellar density of the faint dwarf galaxies (one star per million cubic parsecs) is about a million times lower than that in the neighborhood of the Sun, or almost a billion times lower than in the bulge of the Milky Way.

    As a result, “inhabitants of worlds in one of our NGFS ultra-faint dwarfs would find their sky sparsely populated with visible objects and extremely boring. They would perhaps not even realize that they live in a galaxy!” mused coauthor Thomas Puzia.

    The large number of dwarf galaxies discovered in the Fornax cluster echoes the emerging census of satellites of our own Galaxy, the Milky Way. More than 20 dwarf galaxy companions have been discovered in the past year, many of which were also discovered with DECam.

    Reference: “Unveiling a Rich System of Faint Dwarf Galaxies in the Next Generation Fornax Survey,” Roberto P. Muñoz et al., 2015 November 1, Astrophysical Journal Letters [http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L15, preprint: http://arxiv.org/abs/1510.02475%5D.

    Cerro Tololo Inter-American Observatory is managed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

    See the full article here .

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    NOAO News
    NOAO is the US national research & development center for ground-based night time astronomy. In particular, NOAO is enabling the development of the US optical-infrared (O/IR) System, an alliance of public and private observatories allied for excellence in scientific research, education and public outreach.

    Our core mission is to provide public access to qualified professional researchers via peer-review to forefront scientific capabilities on telescopes operated by NOAO as well as other telescopes throughout the O/IR System. Today, these telescopes range in aperture size from 2-m to 10-m. NOAO is participating in the development of telescopes with aperture sizes of 20-m and larger as well as a unique 8-m telescope that will make a 10-year movie of the Southern sky.

    In support of this mission, NOAO is engaged in programs to develop the next generation of telescopes, instruments, and software tools necessary to enable exploration and investigation through the observable Universe, from planets orbiting other stars to the most distant galaxies in the Universe.

    To communicate the excitement of such world-class scientific research and technology development, NOAO has developed a nationally recognized Education and Public Outreach program. The main goals of the NOAO EPO program are to inspire young people to become explorers in science and research-based technology, and to reach out to groups and individuals who have been historically under-represented in the physics and astronomy science enterprise.

    The National Optical Astronomy Observatory is proud to be a US National Node in the International Year of Astronomy, 2009.

    About Our Observatories:
    Kitt Peak National Observatory (KPNO)

    Kitt Peak

    Kitt Peak National Observatory (KPNO) has its headquarters in Tucson and operates the Mayall 4-meter, the 3.5-meter WIYN , the 2.1-meter and Coudé Feed, and the 0.9-meter telescopes on Kitt Peak Mountain, about 55 miles southwest of the city.

    Cerro Tololo Inter-American Observatory (CTIO)

    NOAO Cerro Tolo

    The Cerro Tololo Inter-American Observatory (CTIO) is located in northern Chile. CTIO operates the 4-meter, 1.5-meter, 0.9-meter, and Curtis Schmidt telescopes at this site.

    The NOAO System Science Center (NSSC)

    Gemini North
    Gemini North

    Gemini South telescope
    Gemini South

    The NOAO System Science Center (NSSC) at NOAO is the gateway for the U.S. astronomical community to the International Gemini Project: twin 8.1 meter telescopes in Hawaii and Chile that provide unprecendented coverage (northern and southern skies) and details of our universe.

    NOAO is managed by the Association of Universities for Research in Astronomy under a Cooperative Agreement with the National Science Foundation.

     
  • richardmitnick 3:45 pm on September 29, 2015 Permalink | Reply
    Tags: , , , Dwarf galaxies   

    From AAS NOVA: ” Killing Star Formation in Satellite Galaxies” 

    AASNOVA

    Amercan Astronomical Society

    14 August 2015
    Susanna Kohler

    1
    The Local Group of galaxies. Dwarf galaxies within the halos of the Milky Way and Andromeda are primarily observed to be quiescent, in contrast to isolated dwarf galaxies. Credit: Andrew Z. Colvin

    When a dwarf galaxy falls into the halo of a large galaxy like the Milky Way, how is star formation in the dwarf affected?

    3
    The Large Magellanic Cloud, a satellite galaxy of the Milky Way. Picture taken by Hubble

    NASA Hubble Telescope
    NASA/ESA Hubble

    A collaboration led by Andrew Wetzel (California Institute of Technology and Carnegie Observatories) recently set out to answer this question using observations of nearby galaxies and simulations of the infall process.

    Observed Quenching

    Isolated dwarf galaxies tend to be gas-rich and very actively star-forming. In contrast, most dwarf galaxies within 300 kpc of us (the Milky Way’s virial radius) contain little or no cold gas, and they’re quiescent: there’s not much star formation happening.

    And this isn’t just true of the Milky Way; we observe the same difference in the satellite galaxies surrounding Andromeda galaxy.

    4
    Andromeda galaxy. Adam Evans

    Once a dwarf galaxy has moved into the gravitational realm of a larger galaxy, the satellite’s gas vanishes rapidly and its star formation is shut off — but how, and on what timescale?

    2
    The known dwarf galaxies in the Local Group (out to 1.6 Mpc) are plotted by their distance from their host vs. their stellar mass. Blue stars indicate actively star-forming dwarfs and red circles indicate quiescent ones. Credit: Wetzel et al. 2015.

    Timescales for Quiescence

    To answer these questions, the authors explored the process of galaxy infall using Exploring the Local Volume in Simulations (ELVIS), a suite of cosmological N-body simulations intended to explore the Local Group. They combined the infall times from the simulations with observational knowledge of the fraction of nearby galaxies that are currently quiescent, in order to determine what timescales are required for different processes to deplete the gas in the dwarf galaxies and quench star formation.

    Based on their results, two types of quenching culprits are at work: gas consumption (where a galaxy simply uses up its immediate gas supply and doesn’t have access to more) and gas stripping (where external forces like ram pressure remove gas from the galaxy).

    These processes operate at different rates for different sizes of galaxies. The authors argue that for galaxies with stellar mass larger than 109 solar masses, the primary means of quenching is gas consumption. The timescale for this mechanism to quench the largest galaxies is roughly 5 Gyr. For galaxies with stellar mass smaller than 109 solar masses, gas stripping takes over, and star-formation is quenched within 1 Gyr for the smallest galaxies.

    Neither quenching mechanisms operates efficiently for galaxies with stellar mass right around 109 solar masses, though, so these galaxies can sustain star formation for much longer. This could explain why the Magellanic clouds (which both have stellar mass of roughly 109 solar masses) are still star-forming despite being within the Milky Way’s halo!

    Citation

    Andrew R. Wetzel et al. 2015 ApJ 808 L27 doi:10.1088/2041-8205/808/1/L27

    See the full article here .

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