Exploring Halo Substructure with Giant Stars: The Velocity Dispersion Profiles of the Ursa Minor and Draco Dwarf Spheroidal Galaxies at Large Angular Separations

The Astrophysical Journal (Impact Factor: 6.28). 12/2008; 631(2):L137. DOI: 10.1086/497396
Source: arXiv

ABSTRACT We analyze velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal (dSph) galaxies based on published and new Keck HIRES spectra for stars in the outer UMi field. Washington + DDO51 filter photometric catalogs provide additional leverage on membership of individual stars and, beyond 0.5 King limiting radii (rlim), identify bona fide dSph members up to 4.5 times more efficiently than simple color-magnitude diagram selections. Previously reported "cold populations" at rlim are not obvious in the data and appear only with particular binning; more or less constant and platykurtic dispersion profiles are characteristic of these dSphs to large radii. We report the discovery of UMi stars to at least 2.7rlim (i.e., 210' or 4 kpc). Even with conservative assumptions, a UMi mass of M > 4.9 × 108 M☉ is required to bind these stars, implying an unlikely global mass-to-light ratio of M/L > 900 (M/L)☉. We conclude that we have found stars tidally stripped from UMi.

  • [Show abstract] [Hide abstract]
    ABSTRACT: The Milky Way (MW) dwarf spheroidal (dSph) satellites are known to be the most dark-matter (DM) dominated galaxies with estimates of dark to baryonic matter reaching even above one hundred. It comes from the assumption that dwarfs are dynamically supported by their observed velocity dispersions. However their spatial distributions around the MW is not at random and this could challenge their origin, previously assumed to be residues of primordial galaxies accreted by the MW potential. Here we show that alternatively, dSphs could be the residue of tidal dwarf galaxies (TDGs), which would have interacted with the Galactic hot gaseous halo and disk. TDGs are gas-rich and have been formed in a tidal tail produced during an ancient merger event at the M31 location, and expelled towards the MW. Our simulations show that low-mass TDGs are fragile to an interaction with the MW disk and halo hot gas. During the interaction, their stellar content is progressively driven out of equilibrium and strongly expands, leading to low surface brightness feature and mimicking high dynamical M/L ratios. Our modeling can reproduce the properties, including the kinematics, of classical MW dwarfs within the mass range of the Magellanic Clouds to Draco. An ancient gas-rich merger at the M31 location could then challenge the currently assumed high content of dark matter in dwarf galaxies. We propose a simple observational test with the coming GAIA mission, to follow their expected stellar expansion, which should not be observed within the current theoretical framework.
    Monthly Notices of the Royal Astronomical Society 04/2014; 442(3). · 5.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The observed central densities of Milky Way dwarf spheroidal galaxies (dSphs) are significantly lower than the densities of the largest (Vmax ˜ 35 km s-1) subhaloes found in dissipationless simulations of Galaxy-size dark matter hosts. One possible explanation is that gas removal from feedback can lower core densities enough to match observations. We model the dynamical effects of supernova feedback through the use of a time-varying central potential in high-resolution, idealized numerical simulations and explore the resulting impact on the mass distributions of dwarf dark matter haloes. We find that in order to match the observed central masses of M⋆ ˜ 106 M⊙ dSphs, the energy equivalent of more than 40 000 supernovae must be delivered with 100 per cent efficiency directly to the dark matter. This energy requirement exceeds the number of supernovae that have ever exploded in most dSphs for typical initial mass functions. We also find that, per unit energy delivered and per cumulative mass removed from the galaxy, single blowout events are more effective than repeated small bursts in reducing central dark matter densities. We conclude that it is unlikely that supernova feedback alone can solve the `too-big-to-fail' problem for Milky Way subhaloes.
    Monthly Notices of the Royal Astronomical Society 08/2013; 433(4):3539-3546. · 5.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We perform numerical simulations to study a formation scenario for dwarf spheroidal (dSph) galaxies in which their stellar populations are the products of the dissolution of open star clusters and stellar associations within cosmological dark matter haloes. This paper shows that this process gives rise to objects which resemble the observed dSph satellites of the Milky Way without invoking external influences. The presence of long-lived kinematic substructures within the stellar components of these objects affects their projected velocity dispersions. We find that this in turn affects mass estimates based on the projected velocity dispersion profiles which may overestimate the actual dark matter halo mass depending on the amount of substructure which is present. Our models make predictions about the detailed kinematic and photometric properties of the dSphs which can be tested using future observations.
    Monthly Notices of the Royal Astronomical Society 06/2013; 432(1):274-284. · 5.23 Impact Factor

Full-text (2 Sources)

Available from
May 23, 2014