Identifying the progenitor set of present-day early-type galaxies: a
  view from the standard model
  We present a comprehensive theoretical study, using a semi-analytical model
within the standard LCDM framework, of the photometric properties of the
progenitors of present-day early-type galaxies in the redshift range 0<z<1. We
explore progenitors of all morphologies and study their characteristics as a
function of the luminosity and local environment of the early-type remnant at
z=0. In agreement with previous studies, we find that, while larger early-types
are generally assembled later, their luminosity-weighted stellar ages are
typically older. In dense cluster-like environments, 70% of early-type systems
are `in place' by z=1 and evolve without interactions thereafter, while in the
field the corresponding value is 30%. Averaging across all environments at z~1,
less than 50% of the stellar mass which ends up in early-types today is
actually in early-type progenitors at this redshift, in agreement with recent
observational work. We develop probabilistic prescriptions which provide a
means of including spiral (i.e. non early-type) progenitors at intermediate and
high redshifts, based on their luminosity and optical colours. For example, we
find that, at intermediate redshifts (z~0.5), large (M_V<-21.5), red (B-V>0.7)
spirals have 75-95% chance of being an early-type progenitor, while the
corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is
50-75%. The prescriptions developed here can be used to address, from the
perspective of the standard model, the issue of `progenitor bias', whereby the
exclusion of late-type progenitors in observational studies can lead to
inaccurate conclusions regarding the evolution of the early-type population
over cosmic time. (abridged)
