Evolution of dwarf spheroidal satellites in the common surface-density dark halos

Abstract

We investigate the growth histories of dark matter halos associated with dwarf satellites in Local Group galaxies and the resultant evolution of the baryonic component. Our model is based on the recently proposed property that the mean surface density of a dark halo inside a radius at maximum circular velocity V_max is universal over a large range of V_max. Following that this surface density of 20 Msun/pc^2 well explains dwarf satellites in the Milky Way and Andromeda, we find that the evolution of the dark halo in this common surface-density scale is characterized by the rapid increase of the halo mass assembled by the redshift z_TT of the tidal truncation by its host halo, at early epochs of z_TT >~ 6 or V_max <~ 22 km/s. This mass growth of the halo is slow at lower z_TT or larger V_max. Taking into account the baryon content in this dark halo evolution, under the influence of the ionizing background radiation, we find that the dwarf satellites are divided into roughly two families: those with V_max <~ 22 km/s having high star formation efficiency and those with larger V_max having less efficient star formation. This semi-analytical model is in good agreement with the high-resolution numerical simulation for galaxy formation and with the observed star formation histories for Fornax and Leo~II. This suggests that the evolution of a dark halo plays a key role in understanding star formation histories in dwarf satellites.