Luminosity functions of XMM-LSS C1 galaxy clusters

Canada-France-Hawaii Telescope Legacy Survey optical photometry has been used to study the galaxy luminosity functions (LFs) of 14 X-ray selected clusters from the XMM Large Scale Survey (XMM-LSS) survey. These are mostly groups and poor clusters, with masses (M₅₀₀) in the range 0.6 to 19 × 10¹³M_solar and redshifts 0.05 <= z <= 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 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, α, but did not always fit well to the bright end. Derived values of α ranged from -1.03 to as steep as -2.1. We find no evidence for upturns at faint magnitudes. Evolution in α 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 ~ 0.3α 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 0. We also calculated the total optical luminosity and found it to correlate strongly with X-ray luminosity (L_X ~ L^2.1_OPT), and also with ICM temperature (L_OPT ~ 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.