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  • richardmitnick 5:40 am on October 4, 2017 Permalink | Reply
    Tags: , , , , , , Dark matter haloes, ESA e-ASTROGAM, , ,   

    From astrobites: “Future Gamma-ray Telescopes and the Search for Dark Matter” 

    Astrobites bloc

    Astrobites

    Oct 3, 2017
    Nora Shipp

    Title: Resolving Dark Matter Subhalos With Future Sub-GeV Gamma-Ray Telescopes
    Authors: Ti-Lin Chou, Dimitrios Tanoglidis, and Dan Hooper
    First Author’s Institution: Dept. of Physics, University of Chicago, USA

    Status: Submitted to the Journal of Cosmology and Astroparticle Physics (open access)

    We are surrounded by undetected dark matter.

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    In fact, our entire Galaxy is enveloped in a large halo of it, but because dark matter does not emit or reflect light, the halo is completely invisible.

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

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Inside this halo, orbiting our galaxy, are hundreds of smaller, equally invisible dark matter halos (Figure 1).

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    Figure 1. Galaxies like the Milky Way are surrounded by small dark matter halos (blue blobs). Some of these halos contain no stars, but could still produce gamma-rays from dark matter annihilation! Source: ESO

    The larger ones contain their own dwarf galaxies, but the smallest halos are so tiny that they contain no stars at all. However, if the leading theory of WIMP (Weakly Interacting Massive Particle) dark matter is correct, there is one way that we could actually see these dark matter halos without the help of any stars. If dark matter particles are their own antiparticle, they would annihilate when they come into contact with each other, producing various particles, including highly energetic photons known as gamma-rays.

    Gamma-rays have millions of times more energy than the optical photons that human eyes can see, yet these energetic particles are quite difficult to detect. The current leader in gamma-ray detection is the Fermi Gamma-ray Space Telescope, a satellite that has been orbiting the Earth, searching the sky for gamma-rays, for almost 10 years.

    NASA/Fermi Telescope

    NASA/Fermi LAT

    Since Fermi was first launched, scientists have searched the gamma-ray sky for evidence of dark matter annihilation. What makes this search really tricky is that dark matter is not the only thing that produces gamma-rays. The sky is actually full of gamma-rays coming from all directions, produced by clouds of gas, pulsars, and active galactic nuclei, among many other sources (Figure 2 [not shown in article, replaced here).

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    Fermi’s Latest Gamma-ray Census Highlights Cosmic Mysteries

    However, those tiny dark matter halos that don’t contain stars or gas or any kind of non-dark matter should only be producing gamma-rays from dark matter annihilation. The catch is that we have no idea where these dark matter halos are. Scientists, therefore, have searched all across the sky for gamma-rays that might be coming from dark halos, and they just might have found a couple.

    Two sources of gamma-rays fit all the requirements – they are in the right part of the sky, do not emit any other kind of light (as you’d expect from a halo containing only dark matter), and appear to extend wider across the sky than the single point of a far away star. However, it’s impossible to tell whether these sources are really extended like a dark matter halo or whether they are just two star-like points so close to each other that they blur together, appearing to Fermi as a single blob. Today’s paper considers whether a proposed successor to Fermi called e-ASTROGAM (Figure 3) will be able to resolve the mystery of these gamma-ray blobs.

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    Figure 3. A model of e-Astrogam, one potential successor to the Fermi Gamma-ray Space Telescope. Source: ESA

    Are they in fact dark matter halos (in which case this would be the first confirmed detection of dark matter annihilation!) or are they simply two points blurred into one?

    e-ASTROGAM would be quite similar to Fermi, but with several important changes. The biggest difference is that it would be able to detect gamma-rays at a slightly lower energy than Fermi, giving us a brand new view of the gamma-ray sky. In the context of today’s paper, however, the most significant difference is the angular resolution. Angular resolution determines how close together two objects can get before they blur together into a single blob. The angular resolution of e-ASTROGAM will be about 4-6 times better than Fermi’s in the energy range of these mysterious gamma-ray sources. According to the authors of today’s paper, this should definitely be enough to tell whether they are single extended objects or two independent points that are just too close together for Fermi to see (Figure 4).

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    Figure 4. Simulated images of two point sources as seen by Fermi and e-ASTROGAM. On the left, Fermi is unable to distinguish between the two objects, seeing only a single blob of gamma-rays. On the right, e-ASTROGAM, with its superior angular resolution, can tell that the single blob is actually two individual objects. Source: Figure 3 of the paper.

    In order to see just how well e-ASTROGAM will be able to see these objects, the authors modeled fake observations of dark matter annihilation from an extended halo and from two point sources. They determined for different halo sizes and dark matter particles how well e-ASTROGAM will be able to tell whether an object is one extended source or two points. Figure 5 illustrates the difference e-ASTROGAM will make in confirming the nature of these gamma-ray sources. The green and red lines represent how easily Fermi and e-ASTROGAM can distinguish pairs of sources (x-axis) as a function of source brightness (y-axis). e-ASTROGAM reaches much farther along the x-axis, indicating that it can much more easily resolve two point sources. The precise numbers change for different dark matter halos and particles, but in all cases e-ASTROGAM shows a significant improvement over Fermi.

    6
    Figure 5. This plot illustrates how e-ASTROGAM will be able to help distinguish between extended dark matter halos and two nearby points. The y-axis shows how bright the object in question is, and the x-axis is related to how easily the telescope can distinguish between two nearby points and a single extended object. Even with really bright objects Fermi (green) has a hard time distinguishing between the two scenarios, while e-ASTROGAM (red) can more easily tell the difference. Source: Figure 5 of the paper.

    A future gamma-ray telescope like e-ASTROGAM will be an essential tool in determining whether Fermi has in fact detected dark matter annihilation from dark halos. In addition to determining whether the two potential halos detected by Fermi are actually just pairs of close-together point sources, e-ASTROGAM may be able to detect gamma-rays from even more dark matter halos that are too faint for Fermi to observe on its own. e-ASTROGAM with its superior angular resolution and lower energy range would provide a brand new view of the gamma-ray universe, giving us unexpected insight into known and unknown sources of gamma-rays, and perhaps finally revealing the nature of dark matter.

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

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

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    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.

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    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).

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    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.

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    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.

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    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.

     
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