Introducing a new, robust galaxy finder algorithm for simulations
  Identifying galaxies in hydrodynamical simulations is a difficult task,
particularly in regions of high density such as galaxy groups and clusters. We
present a new scale-free shape-independent algorithm to robustly and accurately
identify galaxies in simulation, implemented within the phase-space halo-finder
code VELOCIraptor. This is achieved by using the full phase-space dispersion
tensor for particle assignment and an iterative adjustment of search
parameters, which help us overcome common structure finding problems. We apply
our improved method to the Horizon-AGN simulation and compare galaxy stellar
masses ($M_*$), star formation rates (SFR) and sizes with the elaborate
configuration-space halo finder, HaloMaker. Galaxies living in halos with $> 1$
galaxy are the most affected by the shortcomings of real-space finders, with
their mass, SFR, and sizes being $> 2$ times larger (smaller) in the case of
host (satellite) galaxies. Thus, our ability to measure minor/major merger
rates and disentangle environmental effects in simulations can be generally
hindered if the identification of galaxies is not treated carefully. Though
large systematic differences are obtained on a one-to-one basis, the overall
Galaxy Stellar Mass Function, the Star Formation Rate Function and mass-size
relations are not greatly affected. This is due to isolated galaxies being the
most abundant population, dominating broad statistics.
