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RE: Milky Way Companions
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If you want to be eye-catching, bigger is usually better. Not so for dwarf galaxies. It seems that if dwarfs have been fattened up by decaying dark matter, they are harder to spot. This could explain a decades-old puzzle over why we have spotted so few dwarfs circling the Milky Way and nearby giants such as Andromeda.
Astronomers have so far found 20 or so dwarf galaxies in the vicinity of the Milky Way. This number is about 20 times fewer than is predicted by our best model to describe the large-scale structure of the observed universe since the big bang.
According to this model, first to form after the big bang are small clumps, or haloes, of dark matter. These then collide to form bigger and bigger haloes, around which gas coalesces to form stars, galaxies and clusters.

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Title: The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites
Authors: Manuel Metz, Pavel Kroupa, Noam I. Libeskind

Available proper motion measurements of Milky Way (MW) satellite galaxies are used to calculate their orbital poles and projected uncertainties. These are compared to a set of recent cold dark-matter (CDM) simulations, tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the disc-of-satellites of the Milky Way is rotationally supported. Furthermore, the bootstrapping analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution than the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being DM dominated sub-structures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.

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Title: Asymptotic Giant Branch Stars in the Phoenix Dwarf Galaxy
Authors: John Menzies, Michael Feast, Patricia Whitelock, Enrico Olivier, Noriyuki Matsunaga, Gary Da Costa

JHKs near-infrared photometry of stars in the Phoenix dwarf galaxy is presented and discussed. Combining these data with the optical photometry of Massey et al. allows a rather clean separation of field stars from Phoenix members. The discovery of a Mira variable (P = 425 days), which is almost certainly a carbon star, leads to an estimate of the distance modulus of 23.100.18 that is consistent with other estimates and indicates the existence of a significant population of age ~2 Gyr. The two carbon stars of Da Costa have M{bol} = -3.8 and are consistent with belonging to a population of similar age; some other possible members of such a population are identified. A Da Costa non-carbon star is Delta Ks~0.3 mag brighter than these two carbon stars. It may be an AGB star of the dominant old population. The nature of other stars lying close to it in the Ks,(J-Ks) diagram needs studying.

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Title: On the newly discovered Canes Venatici II dSph galaxy
Authors: C. Greco (1,2), M. Dall'Ora (3), G. Clementini (1), V. Ripepi (3), L. Di Fabrizio (4), K. Kinemuchi (5), M. Marconi (3), I. Musella (3), H. A. Smith (6), C. T. Rodgers (7), C. Kuehn (6), T. C. Beers (6,8), M. Catelan (9), B. J. Pritzl (10) ((1) INAF, Osservatorio Astronomico di Bologna, Bologna, Italy; (2) Current address: Observatoire de Geneve, Sauverny, Switzerland; (3) INAF, Osservatorio Astronomico di Capodimonte, Napoli, Italy; (4) INAF, Centro Galileo Galilei & Telescopio Nazionale Galileo, S.Cruz de La Palma, Spain; (5) Universidad de Concepcion, Departamento de Fisica, Concepcion, Chile, and University of Florida, Department of Astronomy, Gainesville, FL, USA; (6) Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA; (7) University of Wyoming, Department of Physics & Astronomy, Laramie, WY, US; (8) Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI, USA; (9) Pontificia Universidad Catolica de Chile, Departamento de Astronomia y Astrofisica, Santiago, Chile; (10) Department of Physics and Astronomy, University of Wisconsin Oshkosh, Oshkosh, WI, USA)

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, B-V colour-magnitude diagram that reaches V = 25.5 mag, showing the galaxy's main sequence turn off at V = 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II,a fundamental-mode variable (RRab) with period P_ab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with P_c = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, <V> = 21.48 0.02 mag, is used to obtain a precision distance modulus of mu_0 = 21.02 0.06 mag and a corresponding distance of 160(+4,-5} kpc, for an adopted reddening E(B-V) = 0.015 mag.

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Title: The Magellanic impact: Collision between the outer Galactic HI disk and the leading arms of the Magellanic stream
Authors: Kenji Bekki, Masashi Chiba, N. M. McClure-Griffiths

We show that collisions between the outer Galactic HI disk and the leading arms (LAs) of the Magellanic stream (MS) can create giant HI holes and chimney-like structures in the disk. Based on the results of our N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we investigate when and where the LAs can pass through the Galactic plane after the MS formation. We then investigate hydrodynamical interaction between LAs and the Galactic HI disk (''the Magellanic impact'') by using our new hydrodynamical simulations with somewhat idealised models of the LAs. We find that about 1-3% of the initial gas mass of the SMC, which consists of the LAs, can pass through the outer part (R=20-35 kpc) of the Galactic HI disk about 0.2 Gyr ago. We also find that the Magellanic impact can push out some fraction (~1%) of the outer Galactic HI disk to form 1-10 kpc-scale HI holes and chimney-like bridges between the LAs and the disk.

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Sagittarius Dwarf galaxy
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Title: Observations of the Sagittarius Dwarf galaxy by the H.E.S.S. experiment and search for a Dark Matter signal
Authors: HESS Collaboration: F. Aharonian, et al

Observations of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy were carried out with the H.E.S.S. array of four imaging air Cherenkov telescopes in June 2006. A total of 11 hours of high quality data are available after data selection. There is no evidence for a very high energy gamma-ray signal above the energy threshold at the target position. A 95% C.L. flux limit of 3.6 x 10-12 cm-2s-1 above 250 GeV has been derived. Constraints on the velocity-weighted cross section <sigma v> are calculated in the framework of Dark Matter particle annihilation using realistic models for the Dark Matter halo profile of Sagittarius dwarf galaxy. Two different models have been investigated encompassing a large class of halo types. A 95% C.L. exclusion limit on <sigma v> of the order of 2 x 10-25 cm3s-1 is obtained for a core profile in the 100 GeV - 1 TeV neutralino mass range.

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RE: Milky Way Companions
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Title: A deep Large Binocular Telescope view of the Canes Venatici I dwarf galaxy
Authors: Nicolas F. Martin, Matthew G. Coleman, Jelte T. A. De Jong, Hans-Walter Rix, Eric F. Bell, David J. Sand, John M. Hill, David Thompson, Vadim Burwitz, Emanuele Giallongo, Roberto Ragazzoni, Emiliano Diolaiti, Federico Gasparo, Andrea Grazian, Fernando Pedichini, Jill Bechtold
(Version v2)

We present the first deep color-magnitude diagram of the Canes Venatici I (CVnI) dwarf galaxy from observations with the wide field Large Binocular Camera on the Large Binocular Telescope. Reaching down to the main-sequence turnoff of the oldest stars, it reveals a dichotomy in the stellar populations of CVnI: it harbors an old (> 10 Gyr), metal-poor ([Fe/H] ~ -2.0) and spatially extended population along with a much younger (~ 1.4-2.0 Gyr), 0.5 dex more metal-rich, and spatially more concentrated population. These young stars are also offset by 64_{-20}^{+40} pc to the East of the galaxy centre. The data suggest that this young population, which represent ~ 3-5 % of the stellar mass of the galaxy within its half-light radius, should be identified with the kinematically cold stellar component found by Ibata et al. (2006). CVnI therefore follows the behaviour of the other remote MW dwarf spheroidals which all contain intermediate age and/or young populations: a complex star formation history is possible in extremely low-mass galaxies.

DwarkGal_ge22
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Title: Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
Authors: Eric Lagadec (University of Manchester), Albert A. Zijlstra (University of Manchester), Mikako Matsuura (NAOJ), J.W. Menzies (SAAO), Jacco Th. van Loon (Keele), Patricia A. Whitelock (SAAO)
(Version v2)

To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies with the 10-\mu m camera VISIR at the VLT. We derive mass-loss rates for 29 AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated from the K-[9] and K-[11] colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between K-[12] colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e. the Galaxy, the LMC and the SMC.
The derived dust mass-loss rates are in the range 5 x 10^{-10} to 3 x 10^{-8} M_{\odot}yr^{-1} for the observed AGB stars in Sgr dSph and around 5 x 10^{-9} M_{\odot}yr^{-1} for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models.

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Stars in dwarf spheroidal galaxies behave in a way that suggests the galaxies are utterly dominated by dark matter, University of Michigan astronomers have found.
Astronomy professor Mario Mateo and post-doctoral researcher Matthew Walker measured the velocity of 6,804 stars in seven dwarf satellite galaxies of the Milky Way: Carina, Draco, Fornax, Leo I, Leo II, Sculptor and Sextans. They found that, contrary to what Newton's law of gravity predicts, stars in these galaxies do not move slower the farther they are from their galaxy's core.

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Title: Bootes II: Not a Dwarf Galaxy bu Possibly Shed by One
Authors: Jae-Woo Lee, Bruce W. Carney, Haw Cheng

We present moderately deep VI photometry of Bootes II, obtained using the SOAR Telescope. While identified initially as an old, metal-poor, dwarf galaxy, Bootes II appears to be a moderately metal-rich globular cluster with [Fe/H] = -0.7. The slope of the red giant branch suggests that [alpha/Fe] = 0.0, and the turn-off indicates an age of 7 to 9 Gyrs, considerably younger than most globular clusters, but similar to Palomar 12. Like that object, Bootes II lies in the direction of the Sagittarius dwarf's tidal stream, and, with a distance of almost 50 kpc, is probably associated with the more distant portion of the stream, as defined by Belokurov et al. (2006).

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