Luminosity Functions of XMM-LSS C1 Galaxy Clusters
  CFHTLS optical photometry has been used to study the galaxy luminosity
functions of 14 X-ray selected clusters from the XMM-LSS survey. These are
mostly groups and poor clusters, with masses (M_{500}) in the range 0.6 to
19x10 ^{13} M_solar and redshifts 0.05-0.61. Hence these are some of the
highest redshift X-ray selected groups to have been studied. Lower and upper
colour cuts were used to determine cluster members. We derive individual
luminosity functions (LFs) for all clusters as well as redshift-stacked and
temperature-stacked LFs in three filters, g', r' and z', down to M=-14.5. All
LFs were fitted by Schechter functions which constrained the faint-end slope,
alpha, but did not always fit well to the bright end. Derived values of alpha
ranged from -1.03 to as steep as -2.1. We find no evidence for upturns at faint
magnitudes. Evolution in alpha was apparent in all bands: it becomes shallower
with increasing redshift; for example, in the z' band it flattened from -1.75
at low redshift to -1.22 in the redshift range z=0.43-0.61. Eight of our
systems lie at z~0.3, and we combine these to generate a galaxy LF in three
colours for X-ray selected groups and poor clusters at redshift 0.3. We find
that at z~0.3 alpha is steeper (-1.67) in the green (g') band than it is
(-1.30) in the red (z') band. This colour trend disappears at low redshift,
which we attribute to reddening of faint blue galaxies from z~0.3 to z~0. We
also calculated the total optical luminosity and found it to correlate strongly
with X-ray luminosity (L_X proportional to L_OPT^(2.1)), and also with ICM
temperature (L_OPT proportional to T^(1.62)), consistent with expectations for
self-similar clusters with constant mass-to-light ratio. We did not find any
convincing correlation of Schechter parameters with mean cluster temperature.
