Moving Mesh Cosmology: Properties of Gas Disks
  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 sub-resolution 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 disks in
GADGET and AREPO haloes with comparable numerical resolution. In a matching
sample of GADGET and AREPO haloes we fit simulated gas disks with exponential
profiles. We find that the cold gas disks formed using the moving mesh approach
have systematically larger disk 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
disks 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 disks formed in massive halos, resolved with 10^6
particles/cells, are still systematically smaller in the GADGET run by a factor
of ~2. The reasons for this are: 1) The excessive heating of haloes close to
the cooling radius due to spurious dissipation of the subsonic turbulence in
GADGET; and 2) The efficient delivery of low angular momentum gaseous blobs to
the bottom of the potential well. 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.
