Galaxies in a simulated $\Lambda$CDM Universe I: cold mode and hot cores

We study the formation of galaxies in a (50 Mpc/h)^3 cosmological simulation (2x288^3 particles), evolved using the entropy conserving SPH code Gadget-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary "cold mode" accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code (Keres et al. 2005). Atmospheres of hot, virialized gas develop in halos above ~2.5e11 Msun, a transition mass that is nearly constant from z=3 to z=0. Cold accretion persists in halos above the transition mass, especially at z>=2. It dominates the growth of galaxies in low mass halos at all times, and it is the main driver of the cosmic star formation history. Satellite galaxies have accretion rates similar to central galaxies of the same baryonic mass at high redshifts, but they have less accretion than comparable central galaxies at low redshift. Relative to our earlier results, the Gadget-2 simulations predict much lower rates of "hot mode" accretion from the virialized gas component of massive halos. At z<=1, typical hot accretion rates in halos above 5e12 Msun are below 1 Msun/yr, even though our simulation does not include AGN heating or other forms of "preventive" feedback. The inner density profiles of hot gas in these halos are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these halos by orders of magnitude, so such models may overemphasize the role of preventive feedback in producing observed galaxy masses and colors. A fraction of the massive halos develop cuspy profiles and significant cooling rates between z=1 and z=0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters.