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Koposov S.E.,Institute of Astronomy | Koposov S.E.,Universitetskiy Pr 13 | Gilmore G.,Institute of Astronomy | Walker M.G.,Institute of Astronomy | And 14 more authors.
Astrophysical Journal | Year: 2011

We develop, implement, and characterize an enhanced data reduction approach which delivers precise, accurate, radial velocities from moderate resolution spectroscopy with the fiber-fed VLT/FLAMES+GIRAFFE facility. This facility, with appropriate care, delivers radial velocities adequate to resolve the intrinsic velocity dispersions of the very faint dwarf spheroidal (dSph) galaxies. Importantly, repeated measurements let us reliably calibrate our individual velocity errors (0.2 kms-1 ≤ δV ≤ 5kms -1) and directly detect stars with variable radial velocities. We show, by application to the Boötes I dSph, that the intrinsic velocity dispersion of this system is significantly below 6.5kms-1 reported by previous studies. Our data favor a two-population model of Boötes I, consisting of a majority "cold" stellar component, with velocity dispersion 2.4+0.9 - 0.5kms-1, and a minority "hot" stellar component, with velocity dispersion ∼9 kms -1, although we cannot completely rule out a single component distribution with velocity dispersion 4.60.8 - 0.6kms -1. We speculate that this complex velocity distribution actually reflects the distribution of velocity anisotropy in Boötes I, which is a measure of its formation processes. © 2011. The American Astronomical Society. All rights reserved.

Niederste-Ostholt M.,Institute of Astronomy | Belokurov V.,Institute of Astronomy | Evans N.W.,Institute of Astronomy | Koposov S.,Institute of Astronomy | And 3 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

Using the optimal filter technique applied to Sloan Digital Sky Survey photometry, we have found extended tails stretching about 1° (or several tens of half-light radii) from either side of the ultrafaint globular cluster Palomar 1. The tails contain roughly as many stars as does the cluster itself. Using deeper Hubble Space Telescope data, we see that the isophotes twist in a characteristic S-shape on moving outwards from the cluster centre to the tails. We argue that the main mechanism forming the tails may be relaxation-driven evaporation and that Pal 1 may have been accreted from a now disrupted dwarf galaxy ~500 Myr ago. © 2010 The Authors. Journal compilation © 2010 RAS.

Xue X.-X.,CAS National Astronomical Observatories | Xue X.-X.,Max Planck Institute for Astronomy | Rix H.-W.,Max Planck Institute for Astronomy | Yanny B.,Fermi National Accelerator Laboratory | And 17 more authors.
Astrophysical Journal | Year: 2011

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative "close pair distribution" as a statistic in the four-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20kpc from the Galactic center exhibit stronger substructure than at r gc < 20kpc. © 2011. The American Astronomical Society. All rights reserved.

Koposov S.E.,Max Planck Institute for Astronomy | Koposov S.E.,Institute of Astronomy | Koposov S.E.,Universitetskiy Pr 13 | Rix H.-W.,Max Planck Institute for Astronomy | And 2 more authors.
Astrophysical Journal | Year: 2010

The narrow GD-1 stream of stars, spanning 60° on the sky at a distance of 10kpc from the Sun and 15kpc from the Galactic center, is presumed to be debris from a tidally disrupted star cluster that traces out a test-particle orbit in the Milky Way halo. We combine Sloan Digital Sky Survey (SDSS) photometry, USNO-B astrometry, and SDSS and Calar Alto spectroscopy to construct a complete, empirical six-dimensional (6D) phase-space map of the stream. We find that an eccentric orbit in a flattened isothermal potential describes this phase-space map well. Even after marginalizing over the stream orbital parameters and the distance from the Sun to the Galactic center, the orbital fit to GD-1 places strong constraints on the circular velocity at the Sun's radius Vc = 224 13kms-1 and total potential flattening q Φ = 0.87+0.07 -0.04. When we drop any informative priors on Vc , the GD-1 constraint becomes Vc = 221 18kms-1. Our 6D map of GD-1, therefore, yields the best current constraint on Vc and the only strong constraint on q Φ at Galactocentric radii near R 15kpc. Much, if not all, of the total potential flattening may be attributed to the mass in the stellar disk, so the GD-1 constraints on the flattening of the halo itself are weak: q Φ,halo > 0.89 at 90% confidence. The greatest uncertainty in the 6D map and the orbital analysis stems from the photometric distances, which will be obviated by GAIA. © 2010 The American Astronomical Society.

Maccio A.V.,Max Planck Institute for Astronomy | Kang X.,Max Planck Institute for Astronomy | Fontanot F.,Max Planck Institute for Astronomy | Somerville R.S.,Max Planck Institute for Astronomy | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We study the luminosity function (LF) and the radial distribution of satellite galaxies within Milky Way (MW) sized haloes as predicted in cold dark matter based models of galaxy formation, making use of numerical N-body techniques as well as three different semi-analytic models (SAMs) galaxy formation codes. We extract merger trees from very high-resolution dissipationless simulations of four Galaxy-sized DM haloes, and use these as common input for the SAMs. We present a detailed comparison of our predictions with the observational data recently obtained on the MW satellite LF. We find that SAMs with rather standard astrophysical ingredients are able to reproduce the observed LF over six orders of magnitude in luminosity, down to magnitudes as faint as MV = -2. We also perform a comparison with the actual observed number of satellites as a function of luminosity, by applying the selection criteria of the SDSS survey to our simulations instead of correcting the observations for incompleteness. Using this approach, we again find good agreement for both the luminosity and radial distributions of MW satellites. We investigate which physical processes in our models are responsible for shaping the predicted satellite LF, and find that tidal destruction, suppression of gas infall by a photoionizing background, and supernova feedback all make important contributions. We conclude that the number and luminosity of MW satellites can be naturally accounted for within the (Λ)cold dark matter paradigm, and this should no longer be considered a problem. © 2009 The Authors. Journal compilation © 2009 RAS.

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