Deep Luminosity Functions and Colour-Magnitude Relations for Cluster
  Galaxies at 0.2 < z < 0.6
  We derive deep $I$ band luminosity functions and colour-magnitude diagrams
from HST imaging for eleven $0.2<z<0.6$ clusters observed at various stages of
merging, and a comparison sample of five more relaxed clusters at similar
redshifts. The characteristic magnitude $M^*$ evolves passively out to $z=0.6$,
while the faint end slope of the luminosity function is $\alpha \sim -1$ at all
redshifts. Cluster galaxies must have been completely assembled down to $M_I
\sim -18$ out to $z=0.6$. We observe tight colour-magnitude relations over a
luminosity range of up to 8 magnitudes, consistent with the passive evolution
of ancient stellar populations. This is found in all clusters, irrespective of
their dynamical status (involved in a collision or not, or even within
subclusters for the same object) and suggests that environment does not have a
strong influence on galaxy properties. A red sequence luminosity function can
be followed to the limits of our photometry: we see no evidence of a weakening
of the red sequence to $z=0.6$. The blue galaxy fraction rises with redshift,
especially at fainter absolute magnitudes. We observe bright blue galaxies in
clusters at $z > 0.4$ that are not encountered locally. Surface brightness
selection effects preferentially influence the detectability of faint red
galaxies, accounting for claims of evolution at the faint end.
