Lightcone mock catalogues from semi-analytic models of galaxy formation
  - I. Construction and application to the BzK colour selection
  We introduce a method for constructing end-to-end mock galaxy catalogues
using a semi-analytical model of galaxy formation, applied to the halo merger
trees extracted from a cosmological N-body simulation. The mocks that we
construct are lightcone catalogues, in which a galaxy is placed according to
the epoch at which it first enters the past lightcone of the observer, and
incorporate the evolution of galaxy properties with cosmic time. We determine
the position between the snapshot outputs at which a galaxy enters the
observer's lightcone by interpolation. As an application, we consider the
effectiveness of the BzK colour selection technique, which was designed to
isolate galaxies in the redshift interval 1.4<z<2.5. The mock catalogue is in
reasonable agreement with the observed number counts of all BzK galaxies, as
well as with the observed counts of the subsample of BzKs that are star-forming
galaxies. We predict that over 75 per cent of the model galaxies with
K_{AB}<=23, and 1.4<z<2.5, are selected by the BzK technique. Interloper
galaxies, outside the intended redshift range, are predicted to dominate bright
samples of BzK galaxies (i.e. with K_{AB}<=21). Fainter K-band cuts are
necessary to reduce the predicted interloper fraction. We also show that
shallow B-band photometry can lead to confusion in classifying BzK galaxies as
being star-forming or passively evolving. Overall, we conclude that the BzK
colour selection technique is capable of providing a sample of galaxies that is
representative of the 1.4<z<2.5 galaxy population.
