Luminosity functions of XMM-LSS C1 galaxy clusters

Abstract

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 × 1013 M⊙ 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 ∼ 0.3 α 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 ∝ L2.1OPT), and also with ICM temperature (LOPT ∝ T1.62), 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. © 2009 RAS.