Moving mesh cosmology: tracing cosmological gas accretion

We investigate the nature of gas accretion onto haloes and galaxies at z = 2 us-ing cosmological hydrodynamic simulations run with the moving mesh code AREPO. Implementing a Monte Carlo tracer particle scheme to determine the origin and ther-modynamic history of accreting gas, we make quantitative comparisons to an other-wise identical simulation run with the smoothed particle hydrodynamics (SPH) code GADGET-3. Contrasting these two numerical approaches, we find significant physi-cal differences in the thermodynamic history of accreted gas in massive haloes above ' 1010.5M. In agreement with previous work, GADGET simulations show a cold frac-tion near unity for galaxies forming in massive haloes, implying that only a small percentage of accreted gas heats to an appreciable fraction of the virial temperature during accretion. The same galaxies in AREPO show a much lower cold fraction, for instance < 20 % in haloes with Mhalo ' 1011M. This results from a hot gas accre-tion rate which, at this same halo mass, is an order of magnitude larger than with GADGET, together with a cold accretion rate which is lower by a factor of two. These