Abstract
We report the results of two theoretical studies that examine the dynamics of stellar systems embedded within cold dark matter (CDM) halos in order to assess observational constraints on the dark matter content of Local Group dwarf spheroidals (dSphs). (i) Firstly, approximating the stellar and dark components by King and NFW models, respectively, we calculate the parameters of dark halos consistent with the kinematics and spatial distribution of stars in dSphs as well as with cosmological N‐body simulations. (ii) Subsequently, N‐body realization of these models are constructed to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. The analytical estimates highlight the poor correspondence between luminosity and halo mass. In systems where data exist, the stellar velocity dispersion profiles remains flat almost to the nominal “tidal” radius, implying that stars are deeply embedded within the dwarf halos and are therefore quite resilient to tidal disruption. This is confirmed by our N‐body experiments: halos need to lose more than 90% of their original mass before stars can be stripped. As tidal mass loss proceeds, the stellar luminosity, L, velocity dispersion, σ~0~, central surface brightness, Σ~0~, and core radius, R~c~, decrease monotonically. Remarkably, the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius, which permit us to derive a tidal evolutionary track for each of them. This information is used to examine whether the newly‐discovered ultra‐faintMilkyWay dwarfs are tidally‐stripped versions of the “classical”, bright dwarfs. 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. 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. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)