Dissecting Galaxy Formation: I. Comparison Between Pure Dark Matter and Baryonic Models

Abstract

We compare assembly of DM halos with and without baryons, within the context of cosmological evolution in the LCDM WMAP3 Universe (baryons+DM, BDM model, and pure DM, PDM model). In representative PDM and BDM models we find that baryons contribute decisively to the evolution of the central region, leading to an isothermal DM cusp, and to a flat DM density core -- the result of heating by dynamical friction of the substructure during a quiescent evolution epoch. This process ablates the cold gas from an embedded disk, cutting the star formation rate by ~10, and heats up the spheroidal gas and stellar components, triggering their expansion. The substructure is more resilient in the presence of baryons. The disk which formed from inside-out as gas dominated, is transformed into an intermediate Hubble type by z ~ 2 and to an early type by z ~ 0.5, based on its gas contents and spheroidal-to-disk stellar mass ratio. Only a relatively small ~20% fraction of DM particles in PDM and BDM models are bound within the radius of maximal circular velocity in the halo -- most of the DM particles perform larger radial excursions. We also find that the fraction of baryons within the halo virial radius somewhat increases during the major mergers and decreases during the minor mergers. The net effect appears to be negligible. While the substructure is being tidally-disrupted, mixing of its debris in the halo is not efficient and becomes even less so with z. The streamers formed after z ~ 1 survive largely to the present time -- an important implication for embedded disk evolution.