Moving mesh cosmology: tracing cosmological gas accretion

Abstract

We investigate the nature of gas accretion on to haloes and galaxies at z = 2 using cosmological hydrodynamic simulations run with the moving mesh code arepo. Implementing a Monte Carlo tracer particle scheme to determine the origin and thermodynamic history of accreting gas, we make quantitative comparisons to an otherwise identical simulation run with the smoothed particle hydrodynamics (SPH) code gadget-3. Contrasting these two numerical approaches, we find significant physical differences in the thermodynamic history of accreted gas in massive haloes above ≃1010.5 M⊙. In agreement with previous work, gadget simulations show a cold fraction 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 per cent in haloes with Mhalo ≃ 1011 M⊙. This results from a hot gas accretion 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 2. These discrepancies increase for more massive systems, and we explain both trends in terms of numerical inaccuracies with the standard formulation of SPH. We note, however, that changes in the treatment of interstellar medium physics – feedback, in particular – could modify the observed differences between codes as well as the relative importance of different accretion modes. We explore these differences by evaluating several ways of measuring a cold mode of accretion. As in previous work, the maximum past temperature of gas is compared to either a constant threshold value or some fraction of the virial temperature of each parent halo. We find that the relatively sharp transition from cold- to hot-mode-dominated accretion at halo masses of ≃1011 M⊙ is a consequence of the constant temperature criterion, which can only separate virialized gas above some minimum halo mass. Examining the spatial distribution of accreting gas, we find that the filamentary geometry of accreting gas near the virial radius is a common feature in massive haloes above ≃1011.5 M⊙. Gas filaments in gadget, however, tend to remain collimated and flow coherently to small radii, or artificially fragment and form a large number of purely numerical ‘blobs’. These same filamentary gas streams in arepo show increased heating and disruption at 0.25–0.5rvir and contribute to the hot gas accretion rate in a manner distinct from classical cooling flows.