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RE: Milky Way Companions
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Posts: 131433
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Leo IV dwarf spheroidal galaxy
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Title: The Leo IV dwarf spheroidal galaxy: colour-magnitude diagram and pulsating stars
Authors: M. I. Moretti, M. Dall'Ora, V. Ripepi, G. Clementini, L. Di Fabrizio, H. Smith, N. De Lee, C. Kuehn, M. Catelan, M. Marconi, I. Musella, T. Beers, K. Kinemuchi

We present the first V, B-V colour-magnitude diagram of the Leo IV dwarf spheroidal galaxy, a faint Milky Way satellite recently discovered by the Sloan Digital Sky Survey. We have obtained B,V time-series photometry reaching about half a magnitude below the Leo IV turnoff, which we detect at V= 24.7 mag, and have performed the first study of the variable star population. We have identified three RR Lyrae stars (all fundamental-mode pulsators, RRab) and one SX Phoenicis variable in the galaxy. In the period-amplitude diagram the Leo IV RR Lyrae stars are located close to the loci of Oosterhoff type I systems and the evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. However, their mean pulsation period, <P{ab}>=0.655 days, would suggest an Oosterhoff type II classification for this galaxy. The RR Lyrae stars trace very well the galaxy's horizontal branch, setting its average magnitude at <V_{RR}>= 21.48 0.03 mag (standard deviation of the mean). This leads to a distance modulus of \mu_{0}=20.94 0.07 mag, corresponding to a distance of 154 5 kpc, by adopting for the Leo IV dSph a reddening E(B-V) = 0.04 0.01 mag and a metallicity of [Fe/H] = -2.31 0.10.

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Sagittarius dSph tidal stream
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Title: Old open clusters in the Sagittarius dSph tidal stream -- kith or kin?
Authors: Giovanni Carraro (ESO-Chile), Thomas Bensby (ESO-Chile)

A widely supported formation scenario for the Galactic disc is that it formed inside-out from material accumulated via accretion events. The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is the best example of a such accretion, and its ongoing disruption has resulted in that its stars are being deposited in the Milky Way halo and outer disc. It is therefore appealing to search for possible signatures of the Sgr dSph contribution to the build-up of the Galactic disc. Interestingly, models of the Sgr dSph stream indicate clearly that the trailing tail passes through the outer Galactic disc, at the same galactocentric distance as some anti-centre old open star clusters. We investigate in this Letter the possibility that the two outermost old open clusters, Berkeley~29 and Saurer~1, could have formed inside the Sgr dSph and then left behind in the outer Galactic disc as a result of tidal interaction with the Milky Way. The actual location of these two star clusters, inside the Sgr dSph trailing tail, is compatible with this scenario, and their chemical and kinematical properties, together with our present understanding of the age-metallicity relationship in the Sgr dSph, lends further support to this possible association. Hence, we find it likely that the old open star clusters Berkeley~29 and Saurer~1 have extra-galactic origins.

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Sagittarius dwarf tidal stream
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Title: Tracing Sagittarius Structure with SDSS and SEGUE Imaging and Spectroscopy
Authors: Brian Yanny, Heidi Jo Newberg, Jennifer A. Johnson, Young Sun Lee, Timothy C. Beers, Dmitry Bizyaev, Howard Brewington, Paola Re Fiorentin, Paul Harding, Elena Malanushenko, Viktor Malanushenko, Dan Oravetz, Kaike Pan, Audrey Simmons, Stephanie Snedden
(Version v2)

We show that the Sagittarius dwarf tidal stream can be traced with very red K/M-giant stars selected from SDSS photometry. A subset of these stars are spectroscopically confirmed with SEGUE and SDSS spectra, and the distance scale of 2MASS and SDSS M giants is calibrated to the RR Lyrae distance scale. The absolute g band magnitude of the K/M-giant stars at the tip of the giant branch is M_g=-1.0. The line-of-sight velocities of the M giant and BHB stars that are spatially coincident with the Sgr dwarf tidal stream are consistent with those of previous authors, reinforcing the need for new models that can explain all of the Sgr tidal debris stream observations. We estimate stellar densities along the tidal tails that can be used to help constrain future models. The K/M-giant, BHB, and F-turnoff stars in the lower surface brightness tidal stream that is adjacent to the main leading Sgr dwarf tidal tail have velocities and metallicities that are similar to those of the stars in the leading tidal tail. The ratio of K/M giants to BHBs and BHBs to F-turnoff stars are also similar for both branches of the leading tidal tail. We show that there is an additional low-metallicity tidal stream near the Sgr trailing tidal tail.

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RE: Milky Way Companions
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How small can a galaxy be?
Astronomers are now finding small-fry galaxies that contain fewer than a million, possibly as few as a thousand, stars.
Until recently, these very faint, dwarf galaxies in the halo of the Milky Way have eluded discovery.
Now astronomers are using advanced techniques and instruments at The University of Arizona/Smithsonian 6.5-meter MMT Observatory at Mount Hopkins, Arizona, to find them.
They reported their latest discovery of such a galaxy, in the constellation Aries, in an online preprint last March. Their research article will be published in Monthly Notices, a publication of the Royal Astronomical Society, this summer.

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Grillmair Dionatos Cold Stellar Stream
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Title: An Orbit Fit for the Grillmair Dionatos Cold Stellar Stream
Authors: Benjamin A. Willett, Heidi Jo Newberg, Haotong Zhang, Brian Yanny, Timothy C. Beers

We use velocity and metallicity information from SDSS and SEGUE stellar spectroscopy to fit an orbit to the narrow 63 stellar stream of Grillmair and Dionatos. The stars in the stream have a retrograde orbit with eccentricity e = 0.33 (perigalacticon of 14.4 kpc and apogalacticon of 28.7 kpc) and inclination approximately i ~ 35. In the region of the orbit which is detected, it has a distance of about 7 to 11 kpc from the Sun. Assuming a standard disk plus bulge and logarithmic halo potential for the Milky Way stars plus dark matter, the stream stars are moving with a large space velocity of approximately 276
m km s^{-1} at perigalacticon. Using this stream alone, we are unable to determine if the dark matter halo is oblate or prolate. The metallicity of the stream is [Fe/H] = -2.1 0.1. Observed proper motions for individual stream members above the main sequence turnoff are consistent with the derived orbit. None of the known globular clusters in the Milky Way have positions, radial velocities, and metallicities that are consistent with being the progenitor of the GD-1 stream.

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Dwarf spheroidals
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Title: Discs of Satellites: the new dwarf spheroidals
Authors: Manuel Metz, Pavel Kroupa, Helmut Jerjen

The spatial distributions of the most recently discovered ultra faint dwarf satellites around the Milky Way and the Andromeda galaxy are compared to the previously reported discs-of-satellites (DoS) of their host galaxies. In our investigation we pay special attention to the selection bias introduced due to the limited sky coverage of SDSS. We find that the new Milky Way satellite galaxies follow closely the DoS defined by the more luminous dwarfs, thereby further emphasising the statistical significance of this feature in the Galactic halo. We also notice a deficit of satellite galaxies with Galactocentric distances larger than 100 kpc that are away from the disc-of-satellites of the Milky Way. In the case of Andromeda, we obtain similar results, naturally complementing our previous finding and strengthening the notion that the discs-of-satellites are optical manifestations of a phase-space correlation of satellite galaxies.

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Segue 1
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Astronomers have identified what appears to be the dimmest galaxy in the universe. Although it shines with the brightness of only a few hundred Suns, it seems to be full of dark matter, making it an ideal candidate to search for evidence of the mysterious material, they say.
The galaxy, known as Segue 1, is one of roughly two dozen dwarf galaxies that orbit the Milky Way. It sits close to the Sagittarius stream, a river of stellar debris torn from another dwarf galaxy.


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Leo V
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Title: Leo V: A Companion of a Companion of the Milky Way Galaxy
Authors: V. Belokurov (1), M.G. Walker (1), N.W. Evans (1), D.C. Faria (1), G. Gilmore (1), M.J. Irwin (1), S. Koposov (2), M. Mateo (3), E. Olszewski (4), D. Zucker (1) ((1) Cambridge, (2) MPIA, (3) Michigan, (4) Tucson)

We report the discovery of a new Milky Way dwarf spheroidal galaxy in the constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo V lies at a distance of about 180 kpc, and is separated by about 3 degrees from another recent discovery, Leo IV. We present follow-up imaging from the Isaac Newton Telescope and spectroscopy from the Hectochelle fibre spectrograph at the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s, which is offset by about 40 km/s from that of Leo IV. A simple interpretation of the kinematic data is that both objects may lie on the same stream, though the implied orbit is only modestly eccentric (e = 0.2)

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Posts: 131433
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RE: Milky Way Companions
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Title: The mass content of the Sculptor dwarf spheroidal galaxy
Authors: G. Battaglia, A. Helmi, E. Tolstoy, M. Irwin

We present a new determination of the mass content of the Sculptor dwarf spheroidal galaxy, based on a novel approach which takes into account the two distinct stellar populations present in this galaxy. This method helps to partially break the well-known mass-anisotropy degeneracy present in the modelling of pressure-supported stellar systems.

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