Modeling Star Formation in Dwarf Spheroidal Galaxies: A Case for Extended Dark Matter Halos

We propose a simple model for the formation of dwarf spheroidal galaxies, inwhich stars are assumed to have formed from isothermal gas in hydrostaticequilibrium inside extended dark matter halos. After expelling the leftovergas, the stellar system undergoes a dynamical relaxation inside the dark matterhalo. These models can adequately describe the observed properties of three(Draco, Sculptor, and Carina) out of four Galactic dwarf spheroidal satellitesstudied in this paper. We suggest that the fourth galaxy (Fornax), which cannotbe fitted well with our model, is observed all the way to its tidal radius. Ourbest fitting models have virial masses of ~10^9 M_Sun, halo formation redshiftsconsistent with the age of oldest stars in these dwarfs, and shallow inner darkmatter density profiles (with slope gamma -0.5...0). The inferred temperatureof gas is ~10^4 K. In our model, the "extratidal" stars observed in thevicinity of some dwarf spheroidal galaxies are gravitationally bound to thegalaxies and are a part of the extended stellar halos. The inferred virialmasses make Galactic dwarf spheroidals massive enough to alleviate the "missingsatellites" problem of LCDM cosmologies.Comment: ApJ accepted, 15 pages, 8 figure