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Canes Venatici I
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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, Christopher S. Kochanek, David Thompson, Vadim Burwitz, Emanuele Giallongo, Roberto Ragazzoni, Emiliano Diolaiti, Federico Gasparo, Andrea Grazian, Fernando Pedichini, Jill Bechtold

We present the first deep colour-magnitude diagram of the Canes Venatici I (CVnI) dwarf galaxy from observations with the wide field Large Binocular Camera of 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 harbours 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 ~100 pc to the East of the centre of the galaxy. The data suggest that this young population 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.

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Posts: 131433
Date:
RE: Milky Way Companions
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Title: Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy
Authors: M. Matsuura (1,2), A. A. Zijlstra (3), J. Bernard-Salas (4), J. W. Menzies (5), G. C. Sloan (4), P. A. Whitelock (5,6,7), P. R. Wood (8), M.-R. L. Cioni (9), M. W. Feast (6), E. Lagadec (3), J. Th. van Loon (10), M. A. T. Groenewegen (11), G. J. Harris (2) ((1) NAO of Japan, (2) University College London, (3) University of Manchester, (4) Cornell University, (5) SAAO, (6) Astronomy Department, University of Cape Town, (7) NASSP, University of Cape Town, (8) Mount Stromlo Observatory, (9) University of Edinburgh, (10) Keele University, (11) KU Leuven)

We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal (dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space Telescope. The stars were selected from a near-infrared survey of Fornax and include the three reddest stars, with presumably the highest mass-loss rates, in that galaxy. Such carbon stars probably belong to the intermediate-age population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary aim of this paper is to investigate mass-loss rate, as a function of luminosity and metallicity, by comparing AGB stars in several galaxies with different metallicities. The spectra of three stars are fitted with a radiative transfer model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We find no evidence that these rates depend on metallicity, although we do suggest that the gas-to-dust ratio could be higher than at solar metallicity, in the range 240 to 800. The C2H2 bands are stronger at lower metallicity because of the higher C/O ratio. In contrast, the SiC fraction is reduced at low metallicity, due to low silicon abundance. The total mass-loss rate from all known carbon-rich AGB stars into the interstellar medium of this galaxy is of the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf irregular galaxy WLM, which has a similar visual luminosity and metallicity. The difference is attributed to the younger stellar population of WLM. The suppressed gas-return rate to the ISM accentuates the difference between the relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies. Our study will be useful to constrain gas and dust recycling processes in low metallicity galaxies.

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Posts: 131433
Date:
Canes Venatici I
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Title: Variable Stars in the Newly Discovered Milky Way Dwarf Spheroidal Satellite Canes Venatici I
Authors: Charles Kuehn, Karen Kinemuchi, Vincenzo Ripepi, Gisella Clementini, Massimo Dall'Ora, Luca Di Fabrizio, Christopher T. Rodgers, Claudia Greco, Marcella Marconi, Ilaria Musella, Horace A. Smith, Márcio Catelan, Timothy C. Beers, Barton J. Pritzl

We have identified 23 RR Lyrae stars and 3 possible Anomalous Cepheids among 84 candidate variables in the recently discovered Canes Venatici I dwarf spheroidal galaxy. The mean period of 18 RRab type stars is <Pab> = 0.60 ±0.01 days. This period, and the location of these stars in the period-amplitude diagram, suggest that Canes Venatici I is likely an Oosterhoff-intermediate system. The average apparent magnitude of the RR Lyrae stars <V> = 22.17 ±0.02 is used to obtain a precision distance estimate of 210 +7/-5 kpc, for an adopted reddening E(B-V)=0.03 mag. We present a B,V colour-magnitude diagram of Canes Venatici I that reaches V about 5 mag, and shows that the galaxy has a mainly old stellar population with a metal abundance near [Fe/H] = -2.0 dex. The width of the red giant branch and the location of the candidate Anomalous Cepheids on the colour-magnitude diagram may indicate that the galaxy hosts a complex stellar population with stars from about 13 Gyr to as young as about 0.6 Gyr.

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RE: Milky Way Companions
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Title: The Tidal Evolution of Local Group Dwarf Spheroidals
Authors: Jorge Penarrubia, Julio F. Navarro, Alan W. McConnachie
(Version v2)

We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically-motivated model where dSphs are approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, R_c decreases by just a factor of ~2. Interestingly, tides tend to make dSphs more dark-matter dominated because the tightly bound central dark matter "cusp" is more resilient to disruption than the "cored" King profile. We examine whether the extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have been caused by tidal stripping of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold.

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dwarfgal_2
Credit: M. Geha

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Posts: 131433
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Mauna Kea scientists may have solved a discrepancy between the number of extremely small, faint galaxies predicted to exist near the Milky Way and the number actually observed. In an attempt to resolve the Missing Dwarf Galaxy problem, two astronomers used the W. M. Keck Observatory to study a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest the Missing Dwarf Galaxy problem is not as severe as previously thought, and may have been solved completely.

It seems that very small, ultra-faint galaxies are far more plentiful than we thought.  If you asked me last year whether galaxies this small and this dark existed, I would have said no. Im astonished that so many tiny, dark matter-dominated galaxies have now been discovered - Dr. Marla Geha, co-author of the study and a Plaskett Research Fellow at the Herzberg Institute of Astrophysics in Canada.

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Title: The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs
Authors: Louis E. Strigari, Savvas M. Koushiappas, James S. Bullock, Manoj Kaplinghat, Joshua D. Simon, Marla Geha, Beth Willman

We use kinematic data from three new, nearby, extremely low-luminosity Milky Way dwarf galaxies (Ursa Major II, Willman 1, and Coma Berenices) to constrain the properties of their dark matter halos, and from these make predictions for the gamma-ray flux from annihilation of dark matter particles in these halos. We show that these 10^3 solar luminosity dwarfs are the most dark matter dominated galaxies in the Universe, with total masses within 100 pc in excess of 10^6 solar masses. Coupled with their relative proximity, their large masses imply that they should have mean gamma-ray fluxes comparable to or greater than any other known satellite galaxy of the Milky Way. Our results are robust to both variations of the inner slope of the density profile and the effect of tidal interactions. The fluxes could be boosted by up to two orders of magnitude if we include the density enhancements caused by surviving dark matter substructure.

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Posts: 131433
Date:
SDSS J1257+3419
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Title: The Faint Stellar Object SDSS J1257+3419 is a Dark Matter Dominated System
Authors: Hideyuki Kamaya

A recent study has revealed SDSS J1257+3419 is either a faint and small dwarf galaxy or a faint and widely extended globular cluster. In this Letter, the author suggests this stellar system is a dwarf spheroidal (dSph). Adopting an observational relation between binding energy and mass of old stellar systems, we derive a relation between mass and size of dSphs by assuming that they are dark matter dominated and virialised objects. Letting half-light radius represent size of SDSS J1257+3419, we find that its mass is ~ 7 x 10^6 solar mass. This indicates mass-to-light ratio (M/L) of SDSS J1257+3419 is about 1000 in the solar unit. This large M/L is expected from a Mateo plot of dSphs. Thus, we insist SDSS J1257+3419 is a dSph.

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Posts: 131433
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Sagittarius Dwarf Galaxy
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Title: A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: Variation of the Metallicity Distribution Function Along the Sagittarius Stream
Authors: Mei-Yin Chou, Steven R. Majewski, Katia Cunha, Verne V. Smith, Richard J. Patterson, David Martinez-Delgado, David R. Law, Jeffrey D. Crane, Ricardo R. Munoz, Ramon Garcia Lopez, Doug Geisler, Michael F. Skrutskie
(Version v2)

We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the stars they have contributed to the halo. Our results suggest that Sgr experienced a significant change in binding energy over the past several Gyr, which has substantially decreased its tidal boundary across a radial range over which there must have been a significant metallicity gradient in the progenitor galaxy. By accounting for MDF variation along the debris arms, we approximate the MDF Sgr would have had several Gyr ago. We also analyse the MDF of a moving group of M giants we previously discovered towards the North Galactic Cap having opposite radial velocities to the infalling Sgr leading arm stars there and propose that most of these represent Sgr trailing arm stars overlapping the Sgr leading arm in this part of the sky. If so, these trailing arm stars further demonstrate the strong MDF evolution within the Sgr stream.

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The Magellanic clouds
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The Milky Way's closest galactic neighbours, the Magellanic clouds, are simply paying us a visit rather than being cannibalised, according to a controversial paper.
New measurements show these satellite galaxies are moving too quickly to have been captured and disrupted by the more massive Milky Way.

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