Effects of selection and covariance on X-ray scaling relations of galaxy clusters

Abstract

We explore how the behaviour of galaxy cluster scaling relations are affected by flux-limited selection biases and intrinsic covariance among observable properties. Our models presume log-normal covariance between luminosity (L) and temperature (T) at fixed mass (M), centred on evolving, power-law mean relations as a function of host halo mass. Selection can mimic evolution; the L-M and L-T relations from shallow X-ray flux-limited samples will deviate from mass-limited expectations at nearly all scales while the relations from deep surveys (10-14 erg s-1 cm-2) become complete, and therefore unbiased, at masses above ~2 × 1014H-1M⊙. We derive expressions for low-order moments of the luminosity distribution at fixed temperature, and show that the slope and scatter of the L-T relation observed in flux-limited samples is sensitive to the assumed L-T correlation coefficient. In addition, L-T covariance affects the redshift behaviour of halo counts and mean luminosity in a manner that is nearly degenerate with intrinsic population evolution. © 2007 The Authors. Journal compilation © 2007 RAS.