The Tumultuous Lives of Galactic Dwarfs and the Missing Satellites Problem

Abstract

Hierarchical Cold Dark Matter (CDM) models predict that Milky Way (MW) sized halos contain hundreds of dense low-mass dark satellites, an order of magnitude more than the number of observed satellites in the Local Group (LG). If the CDM paradigm is correct, we need to understand why most of these halos failed to form stars and become galaxies. We analyze the dynamical evolution of the satellite halos in a high-resolution cosmological simulation of MW sized halos in the LCDM cosmology. We find that about 10% of the substructure halos with the present masses <10^8-10^9 Msun (circular velocities Vmax<30 km/s) had considerably larger masses and circular velocities when they formed at redshifts z>2. After the initial period of mass accretion in isolation, these objects experience dramatic mass loss due to tidal stripping, in some cases even before they are accreted by their host halo. This can explain how the smallest dwarf spheroidal galaxies of the LG were able to build up a sizable stellar mass in their seemingly shallow potential wells. We propose a new model in which all of the luminous dwarf spheroidals in the Local Group are descendants of the relatively massive (>10^9 Msun) high-redshift systems, in which the gas could cool efficiently by atomic line emission and which were not significantly affected by the extragalactic ultraviolet radiation. We present a simple galaxy formation model based on the trajectories extracted from the simulation, which accounts for the bursts of star formation after strong tidal shocks and the inefficiency of gas cooling in halos with virial temperatures Tvir<~10^4 K. Our model reproduces the abundance, spatial distribution, and morphological segregation of the observed Galactic satellites. The results are insensitive to the redshift of reionization.