Galaxies in a simulated ΛCDM Universe - I. Cold mode and hot cores

We study the formation of galaxies in a large volume (50 h(-1) Mpc, 2 x 288(3) particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (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. Atmospheres of hot, virialized gas develop in haloes above 2-3 x 10(11) M-circle dot, a transition mass that is nearly constant from z = 3 to 0. Cold accretion persists in haloes above the transition mass, especially at z >= 2. It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at z < 1 the accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, the GADGET-2 simulations predict much lower rates of 'hot mode' accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at z <= 1. Hot accretion is inefficient in haloes above similar to 5 x 10(12) M-circle dot, with typical rates lower than 1M(circle dot) yr(-1) at z <= 1, even though our simulation does not include active galactic nuclei (AGN) heating or other forms of 'preventive' feedback. Instead, the accretion rates are low because the inner density profiles of hot gas in these haloes are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these haloes by orders of magnitude, so these models may overemphasize the role of preventive feedback in producing observed galaxy masses and colours. A fraction of the massive haloes develop cuspy profiles and significant cooling rates between z = 1 and 0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters.