Moving-mesh cosmology: properties of gas discs

We compare the structural properties of galaxies formed in cosmological simulations using the smoothed particle hydrodynamics (SPH) code gadget with those using the moving-mesh code arepo. Both codes employ identical gravity solvers and the same subresolution physics but use very different methods to track the hydrodynamic evolution of gas. This permits us to isolate the effects of the hydro solver on the formation and evolution of galactic gas discs in gadget and arepo haloes with comparable numerical resolution. In a matching sample of gadget and arepo haloes, we fit simulated gas discs with exponential profiles. We find that the cold gas discs formed using the moving-mesh approach have systematically larger disc scale lengths and higher specific angular momenta than their gadget counterparts across a wide range in halo masses. For low-mass galaxies, differences between the properties of the simulated galaxy discs are caused by an insufficient number of resolution elements which lead to the artificial angular momentum transfer in our SPH calculation. We however find that galactic discs formed in massive haloes, resolved with ≥106 particles/cells, are still systematically smaller in the gadget run by a factor of ∼2. The reason for this is twofold: (i) the excessive heating of haloes close to the cooling radius due to spurious dissipation of the subsonic turbulence in gadget reduces the supply of gas which can cool and settle on to the central disc; (ii) the efficient delivery of low angular momentum gaseous blobs to the bottom of the potential well results in the centrally concentrated gas discs in gadget simulation. While this large population of gaseous blobs in gadget originates from the filaments which are pressure confined and fragment due to the SPH surface tension while infalling into hot halo atmospheres, it is essentially absent in the moving-mesh calculation, clearly indicating numerical rather than physical origin of the blob material.