Halo Merger Tree Comparison: Impact on Galaxy Formation Models
  We examine the effect of using different halo finders and merger tree
building algorithms on galaxy properties predicted using the GALFORM
semi-analytical model run on a high resolution, large volume dark matter
simulation. The halo finders/tree builders HBT, ROCKSTAR, SUBFIND and
VELOCIRAPTOR differ in their definitions of halo mass, on whether only spatial
or phase-space information is used, and in how they distinguish satellite and
main haloes; all of these features have some impact on the model galaxies, even
after the trees are post-processed and homogenised by GALFORM. The stellar mass
function is insensitive to the halo and merger tree finder adopted. However, we
find that the number of central and satellite galaxies in GALFORM does depend
slightly on the halo finder/tree builder. The number of galaxies without
resolved subhaloes depends strongly on the tree builder, with VELOCIRAPTOR, a
phase-space finder, showing the largest population of such galaxies. The
distributions of stellar masses, cold and hot gas masses, and star formation
rates agree well between different halo finders/tree builders. However, because
VELOCIRAPTOR has more early progenitor haloes, with these trees GALFORM
produces slightly higher star formation rate densities at high redshift,
smaller galaxy sizes, and larger stellar masses for the spheroid component.
Since in all cases these differences are small we conclude that, when all of
the trees are processed so that the main progenitor mass increases
monotonically, the predicted GALFORM galaxy populations are stable and
consistent for these four halo finders/tree builders.
