nIFTy Cosmology: Comparison of Galaxy Formation Models
  We present a comparison of 14 galaxy formation models: 12 different
semi-analytical models and 2 halo-occupation distribution models for galaxy
formation based upon the same cosmological simulation and merger tree
information derived from it. The participating codes have proven to be very
successful in their own right but they have all been calibrated independently
using various observational data sets, stellar models, and merger trees. In
this paper we apply them without recalibration and this leads to a wide variety
of predictions for the stellar mass function, specific star formation rates,
stellar-to- halo mass ratios, and the abundance of orphan galaxies. The scatter
is much larger than seen in previous comparison studies primarily because the
codes have been used outside of their native environment within which they are
well tested and calibrated. The purpose of the `nIFTy comparison of galaxy
formation models' is to bring together as many different galaxy formation
modellers as possible and to investigate a common approach to model
calibration. This paper provides a unified description for all participating
models and presents the initial, uncalibrated comparison as a baseline for our
future studies where we will develop a common calibration framework and address
the extent to which that reduces the scatter in the model predictions seen
here.
