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Title: Search for Unknown Dark Matter Satellites of the Milky Way
Authors: Alex Drlica-Wagner, Ping Wang, Elliott Bloom, Louis Strigari, for the Fermi-LAT Collaboration

We present a search for Galactic dark matter (DM) satellites using the Large Area Telescope (LAT). N-body simulations based on the Lambda-CDM model of cosmology predict a large number of as yet unobserved Galactic DM satellites. These satellites could potentially produce gamma rays through the self-annihilation of DM particles. Some DM satellites are expected to have hard gamma-ray spectra, finite angular extents, and a lack of counterparts at other wavelengths. We searched for LAT sources with these characteristics. We found no candidate DM satellites matching these criteria in one year of LAT data and interpreted this result in the context of N-body simulations.

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Title: Dwarf spheroidal galaxy kinematics and spiral galaxy scaling laws
Authors: Paolo Salucci, Mark I. Wilkinson, Matthew G. Walker, Gerard F. Gilmore, Eva K. Grebel, Andreas Koch, Christiane Frigerio Martins, Rosemary F.G. Wyse

Kinematic surveys of the dwarf spheroidal (dSph) satellites of the Milky Way are revealing tantalising hints about the structure of dark matter (DM) haloes at the low-mass end of the galaxy luminosity function. At the bright end, modelling of spiral galaxies has shown that their rotation curves are consistent with the hypothesis of a Universal Rotation Curve whose shape is supported by a cored dark matter halo. In this paper, we investigate whether the internal kinematics of the Milky Way dSphs are consistent with the particular cored DM distributions which reproduce the properties of spiral galaxies. Although the DM densities in dSphs are typically almost two orders of magnitude higher than those found in (larger) disk systems, we find consistency between dSph kinematics and Burkert DM haloes whose core radii r0 and central densities {
ho}0 lie on the extrapolation of the scaling law seen in spiral galaxies: log {
ho}0 \simeq {\alpha} log r0 + const with 0.9 < {\alpha} < 1.1. We similarly find that the dSph data are consistent with the relation between {
ho}0 and baryon scale length seen in spiral galaxies. While the origin of these scaling relations is unclear, the finding that a single DM halo profile is consistent with kinematic data in galaxies of widely varying size, luminosity and Hubble Type is important for our understanding of observed galaxies and must be accounted for in models of galaxy formation.

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Dwarf spheroidal satellite galaxies
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Title: Metals Removed by Outflows from Milky Way Dwarf Spheroidal Galaxies
Authors: Evan N. Kirby (1), Crystal L. Martin (2), Kristian Finlator (2) ((1) California Institute of Technology, (2) University of California Santa Barbara)

The stars in the dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are significantly more metal-poor than would be expected from a closed box model of chemical evolution. Gas outflows likely carried away most of the metals produced by the dSphs. Based on previous Keck/DEIMOS observations and models, we calculate the mass in Mg, Si, Ca, and Fe expelled from each of eight dSphs. Essentially, these masses are the differences between the observed amount of metals present in the dSphs' stars today and the inferred amount of metals produced by supernovae. We conclude that the dSphs lost 96% to >99% of the metals their stars manufactured. We apply the observed mass function of Milky Way dSphs to the ejected mass function to determine that a single large dSph, like Fornax, lost more metals over 10 Gyr than all smaller dSphs combined. Therefore, small galaxies like dSphs are not significant contributors to the metal content of the intergalactic medium. Finally, we compare our ejected mass function to previous X-ray measurements of the metal content of the winds from the post-starburst dwarf irregular galaxy NGC 1569. Remarkably, the most recent starburst in that galaxy falls exactly on the ejected mass-stellar mass relation defined by the Milky Way dSphs.

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Milky way's first stars killed the satellite galaxies
              
Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of our galaxy's first stars. More than 12 billion years ago, their intense light dispersed the gas of the Milky Way's satellite galaxies. By computing the observable consequences of this process, Pierre Ocvirk and Dominique Aubert demonstrated their prevailing role. This result confirms that reionisation is indeed an essential process in the standard model of galaxy formation. The study took place within the LIDAU collaboration (Light In the Dark Ages of the Universe). It is published in the October issue of the letters of the Monthly Notices of the Royal Astronomical Society.
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How the Milky Way killed off its satellites

Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of the first stars in our galaxy, the Milky Way. More than 12 billion years ago, the intense ultraviolet light from these stars dispersed the gas of our Galaxy's nearest companions, virtually putting a halt to their ability to form stars and consigning them to a dim future. Now Pierre Ocvirk and Dominique Aubert, members of the Light in the Dark Ages of the Universe (LIDAU) collaboration, have explained why some galaxies were killed off, while stars continued to form in more distant objects. The two scientists publish their results in the October issue of the letters of the journal Monthly Notices of the Royal Astronomical Society.
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Milky Way satellite galaxies
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Title: Infall Times for Milky Way Satellites From Their Present-Day Kinematics
Authors: Miguel Rocha, Annika H.G. Peter, James S. Bullock (Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine)

We analyse subhalos in the Via Lactea II (VL2) cosmological simulation to look for correlations among their infall times and z = 0 dynamical properties. We find that the present day orbital energy is tightly correlated with the time at which subhalos last crossed into the virial radius. This energy-infall correlation provides a means to infer infall times for Milky Way satellite galaxies. Assuming that the Milky Way's assembly can be modelled by VL2, we show that the infall times of some satellites are well constrained given only their Galactocentric positions and line-of-sight velocities. The constraints sharpen for satellites with proper motion measurements. We find that Carina, Ursa Minor, and Sculptor were all accreted early, more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are also probable early accreters, though with larger uncertainties. On the other extreme, Leo T is just falling into the Milky Way for the first time while Leo I fell in ~ 2 Gyr ago and is now climbing out of the Milky Way's potential after its first perigalacticon. The energies of several other dwarfs, including Fornax and Hercules, point to intermediate infall times, 2 - 8 Gyr ago. We compare our infall time estimates to published star formation histories and find hints of a dichotomy between ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched star formation after infall but the ultrafaint dwarfs tend to be quenched long before infall, at least for the cases in which our uncertainties allow us to discern differences. Our analysis suggests that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within the last ~ 4 billion years.

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Title: Making Counter-Orbiting Tidal Debris: The Origin of the Milky Way Disc of Satellites
Authors: M. S. Pawlowski, P. Kroupa, K. S. de Boer

    Using stellar-dynamical calculations it is shown for the first time that counter-orbiting material emerges naturally in tidal interactions of disc galaxies. Model particles on both pro- and retrograde orbits can be formed as tidal debris in single encounters with disc galaxies of 1-to-1 and 4-to-1 mass ratios. A total of 74 model calculations are performed for a range of different initial parameters. Interactions include fly-by and merger cases. The fraction of counter-orbiting material produced varies over a wide range (from a few up to 50 percent). All fly-by models show a similar two-phase behaviour, with retrograde material forming first. Properties of the prograde and retrograde populations are extracted to make an observational discrimination possible.
    During such encounters the tidal debris occupies a certain region in phase space. In this material, tidal-dwarf galaxies may form. The modelling therefore can explain why galaxies may have dwarf galaxies orbiting counter to the bulk of their dwarf galaxies. An example is the Sculptor dwarf of the Milky Way, which orbits counter to the bulk of the disc of satellites. The modelling thus supports the scenario of the MW satellites being ancient tidal-dwarf galaxies formed from gaseous material stripped from another galaxy during an encounter with the young MW.
    A possible candidate for this galaxy is identified as the Magellanic Cloud progenitor galaxy. Its angular motion fits the angular motion of the MW disc of satellites objects. This scenario is in agreement with Lynden-Bell's original suggestion for the origin of the dSph satellites and the near-unbound orbit of the LMC.

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Our understanding of dark matter says the Milky Way should have many times more than its dozen or so small satellite galaxies

Dark matter. Nobody knows what it is, but it's thought to make up a quarter of the universe. If that's so, theory predicts that thousands of dark matter clumps should surround the Milky Way, each holding a small satellite galaxy.
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Known streams in the Milky Way
Name Origin Mass
(Solar masses)
Length
(light years)
Composition Discovery year
Arcturus stream Defunct dwarf galaxy Unknown Unknown Old stars deficient in heavy elements 1971
Magellanic Stream Large and Small Magellanic Clouds 200 million 1 million Hydrogen gas 1972
Sagittarius stream Sagittarius Dwarf Elliptical Galaxy 100 million 1 million Wide variety of stars 1994
Helmi stream Defunct dwarf galaxy 10 to 100 million Several complete loops around the Milky Way Old stars deficient in heavy elements 1999
Palomar 5 stream Globular cluster Palomar 5 5,000 30,000 Old stars 2001
Virgo stream Defunct dwarf galaxy 30,000 2001
Monoceros ring Canis Major dwarf galaxy 100 million 200,000 Intermediate-age stars 2002
Anticenter stream Defunct dwarf galaxy Unknown 30,000 Old stars 2006
NGC 5466 stream

45 Degree Tidal Stream

Globular cluster NGC 5466 10,000 60,000 Very old stars 2006
Orphan stream Ursa Major II Dwarf galaxy 100,000 20,000 Old stars 2006
Acheron stream Globular cluster 2007
Cocytus stream Globular cluster 2007
Lethe stream Globular cluster 2007
Styx stream Defunct dwarf galaxy 2007
Bootes III stream Embedded in, & possible progenitor of Styx stream 2007

Source


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Sagittarius Stream
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Title: Was the Progenitor of the Sagittarius Stream a Disc Galaxy?
Authors: Jorge Penarrubia, Vasily Belokurov, N. Wyn Evans, David Martinez-Delgado, Gerard Gilmore, Mike Irwin, Martin Niederste-Ostholt, Daniel B. Zucker

We use N-body simulations to explore the possibility that the Sagittarius (Sgr) dwarf galaxy was originally a late-type, rotating disc galaxy, rather than a non-rotating, pressure-supported dwarf spheroidal galaxy, as previously thought. We find that bifurcations in the leading tail of the Sgr stream, similar to those detected by the SDSS survey, naturally arise in models where the Sgr disc is misaligned with respect to the orbital plane. Moreover, we show that the internal rotation of the progenitor may strongly alter the location of the leading tail projected on the sky, and thus affect the constraints on the shape of the Milky Way dark matter halo that may be derived from modelling the Sgr stream. Our models provide a clear, easily-tested prediction: although tidal mass stripping removes a large fraction of the original angular momentum in the progenitor dwarf galaxy, the remnant core should still rotate with a velocity amplitude ~20 km/s that could be readily detected in future, wide-field kinematic surveys of the Sgr dwarf.

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