Hunting for Galaxies and Halos in simulations with VELOCIraptor
  We present VELOCIraptor, a massively parallel galaxy/(sub)halo finder that is
also capable of robustly identifying tidally disrupted objects and separate
stellar halos from galaxies. The code is written in c++11, use the MPI and
OpenMP API's for parallelisation, and includes python tools to read/manipulate
the data products produced. We demonstrate the power of the VELOCIraptor
(sub)halo finder, showing how it can identify subhalos deep within the host
that have negligible density contrasts to their parent halo. We find a subhalo
mass-radial distance dependence: large subhalos with mass ratios of
$\gtrsim10^{-2}$ are more common in the central regions that smaller subhalos,
a result of dynamical friction and low tidal mass loss rates. This dependence
is completely absent in (sub)halo finders in common use, which generally search
for substructure in configuration space, yet is present in codes that track
particles belonging to halos as they fall into other halos, such as HBT+.
VELOCIraptor largely reproduces the dependence seen without tracking, finding a
similar radial dependence to HBT+ in well resolved halos from our limited
resolution fiducial simulation.
