The physics of gas accretion onto halos and galaxies: insight from cosmological simulations

The accumulation of matter on halo-scales directly feeds the processes which govern the evolution of galaxies over cosmic time. Gas accretion on these scales, to first order, is set by the balance between gravitation and pressure-support, this balance being non-trivial to model and heavily dependent on the state of each individual system. In a Lambda-CDM universe, we expect the picture for dark matter build-up to be much more steady and predictable, due to the lack of self-interaction. To explore these ideas in a theoretical context, we make use of a collection of 25Mpc EAGLE hydrodynamical simulations to investigate (1) the breakdown of accretion modes over cosmic time (in particular, contrasting gas and dark matter accretion), and (2) the effect of physical models on matter accretion, by comparing accretion to halos on a range of scales and simulations that implement cooling and stellar/AGN feedback to varying extents. We find that dark matter is preferentially accumulated in halos via mergers compared to pristine and recycled cosmological modes of accretion, especially for high mass halos, Mhalo > 10^12.5 Msun at late times. In comparison, baryon accumulation in halos is more likely to be of cosmological origin, rather than mergers. We also find that the breakdown of accretion into different modes is heavily dependent on model physics. Our results have important implications on analytic and semi-analytic treatments of galaxy evolution, highlighting the need to treat dark matter and baryons individually.