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RE: Milky Way Companions
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Title: The Magellanic Bridge: The Nearest Purely Tidal Stellar Population
Authors: Jason Harris

We report on observations of the stellar populations in twelve fields spanning the region between the Magellanic Clouds, made with the Mosaic-II camera on the 4-meter telescope at the Cerro-Tololo Inter-American Observatory. The two main goals of the observations are to characterize the young stellar population (which presumably formed in situ in the Bridge and therefore represents the nearest stellar population formed from tidal debris), and to search for an older stellar component (which would have been stripped from either Cloud as stars, by the same tidal forces which formed the gaseous Bridge). We determine the star-formation history of the young inter-Cloud population, which provides a constraint on the timing of the gravitational interaction which formed the Bridge. We do not detect an older stellar population belonging to the Bridge in any of our fields, implying that the material that was stripped from the Clouds to form the Magellanic Bridge was very nearly a pure gas.

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Our galaxy may be surrounded by a swarm of invisible companions made of dark matter.
After the big bang, dark matter should have formed a halo which then attracted ordinary gas to form stars and galaxies. When Jürg Diemand of the University of California, Santa Cruz, and his team modelled this process, they found that there should be at least 10,000 sub-haloes of dark matter within the halo of the Milky Way. Of these some 120 should have attracted some gas of their own and become dwarf galaxies.
Only 15 dwarf companions of the Milky Way have been found. If the remainder have not become visible dwarf galaxies, they could still be detected by gamma-rays given off as the dark matter particles collide.

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Posts: 131433
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Bootes Dwarf Spheroidal Galaxy
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Title: Absence of HI in the Bootes Dwarf Spheroidal Galaxy
Authors: Jeremy Bailin (1), Alyson Ford (1,2) ((1) Swinburne University, (2) ATNF)

Neutral hydrogen observations towards the Bootes dwarf spheroidal galaxy, a very low luminosity metal-poor Galactic satellite, were obtained using the Parkes Radio Telescope. We do not detect any HI in or around Bootes to a 3sigma upper limit of 180 Msun within the optical half light radius and 8000 Msun within 1.6 kpc. Its HI mass-to-light ratio is less than 0.002 Msun/Lsun, making Bootes one of the most gas-poor galaxies known. Either reionisation severely inhibited gas infall onto the proto-Bootes, or large amounts of gas have been removed by ram pressure and/or tidal stripping. Since Bootes lies on the mass-metallicity fundamental line, this relation and the inefficiency of star formation at the faintest end of the galaxy luminosity function must be partly driven, or at least not disrupted, by extreme gas loss in such low luminosity galaxies. We also do not detect any HI associated with the leading tidal tail of the Sagittarius dSph galaxy, which fortuitously passes through the observed field, to a 3sigma column density limit of 2 x 10^17 cm^-2. This suggests that either the leading gaseous tail is ionised, or the gas in the trailing tail was removed before the current tidal disruption of the parent dSph began.

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RE: Milky Way Companions
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Title: A new view of the dwarf spheroidal satellites of the Milky Way from VLT/FLAMES: Where are the very metal poor stars?
Authors: Amina Helmi, M.J. Irwin, E. Tolstoy, G. Battaglia, V. Hill, P. Jablonka, K. Venn, M. Shetrone, B. Letarte, N. Arimoto, T. Abel, P. Francois, A. Kaufer, F. Primas, K. Sadakane, T. Szeifert

As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) program, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Sextans, Fornax, and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below (Fe/H) ~ -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph's appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

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A large survey, made with ESO's VLT, has shed light on our Galaxy's ancestry. After determining the chemical composition of over 2000 stars in four of the nearest dwarf galaxies to our own, astronomers have demonstrated fundamental differences in their make-up, casting doubt on the theory that these diminutive galaxies could ever have formed the building blocks of our Milky Way Galaxy.

"The chemistry we see in the stars in these dwarf galaxies is just not consistent with current cosmological models. It shows that there is plenty of astronomy to learn in our backyard" - Amina Helmi of the Kapteyn Astronomical Institute in Groningen, The Netherlands, and lead author of the paper presenting the results.

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Ursa Major II
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Title: Is Ursa Major II the Progenitor of the Orphan Stream?
Authors: M. Fellhauer (1), N.W. Evans (1), V. Belokurov (1), D.B. Zucker (1), B. Yanny (2), M.I. Wilkinson (1), G. Gilmore (1), M.J. Irwin (1), D.M. Bramich (1), S. Vidrih (1), P. Hewett (1), T. Beers (3) ((1) Cambridge, (2) FNAL, (3) Michigan)

Prominent in the 'Field of Streams' -- the Sloan Digital Sky Survey map of substructure in the Galactic halo -- is an 'Orphan Stream' without obvious progenitor. In this numerical study, we show a possible connection between the newly found dwarf satellite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as -100 km/s as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream.

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RE: Milky Way Companions
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Title: The Warp and Spiral Arms of the Milky Way
Authors: E.S. Levine, Leo Blitz, Carl Heiles, Martin Weinberg

We examine the outer Galactic HI disk for deviations from the b=0 plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency 1 and 2, all of which grow with Galactocentric radius. A perturbation theory calculation demonstrates that the tidal influence of the Magellanic Clouds creates distortions in the dark matter halo, which combine to produce disk warp amplitudes comparable to the observations. We also construct a detailed map of the perturbed surface density of HI in the outer disk demonstrating that the Galaxy is a non-axisymmetric multi-armed spiral. Overdensities in the surface density are coincident with regions of reduced gas thickness.

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Tidal Streams
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Title: Substructure in Tidal Streams; Tributaries in the Anti-centre Ring
Authors: C. J. Grillmair

We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65 degree-long stellar stream complex previously identified with the Anti-centre or Monoceros Ring. The three-stream complex varies in width from 4 to 6 degrees along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of about 8 km/s. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components which may have included a native population of globular clusters. While the color of the main sequence turn-off is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions.

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RE: Milky Way Companions
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4newdwarfs

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Title: Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
Authors: V. Belokurov, D. B. Zucker, N. W. Evans, J. T. Kleyna, S. Koposov, S. T. Hodgkin, M. J. Irwin, G. Gilmore, M. I. Wilkinson, M. Fellhauer, D. M. Bramich, P. C. Hewett, S. Vidrih, J. T. A. De Jong, J. A. Smith, H.-W. Rix, E. F. Bell, R. F. G. Wyse, H. J. Newberg, P. A. Mayeur, B. Yanny, C. M. Rockosi, O. Y. Gnedin, D. P. Schneider, T. C. Beers, J. C. Barentine, H. Brewington, J. Brinkmann, M. Harvanek, S. J. Kleinman, J. Krzesinski, D. Long, A. Nitta, S. A. Snedden

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies -- one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules -- together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and colour-magnitude diagrams for all five satellites. The morphological features of the colour-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness mu_v >~ 28 mag/sq. arcsec have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between ~40 pc and ~ 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

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