Effects of Formation Epoch Distribution on X-Ray Luminosity and Temperature Functions of Galaxy Clusters

Abstract

We investigate statistical properties of galaxy clusters in the context of hierarchical clustering scenario, taking account of their formation epoch distribution, motivated by the recent finding by Fujita and Takahara that X-ray clusters form a fundamental plane, where the mass and formation epoch are regarded as two independent parameters. Using the formalism which discriminates between major merger and accretion, the epoch of a cluster formation is identified with that of the last major merger. Since tiny mass accretion after the formation does not much affect the core structure of clusters, the properties of X-ray emission from clusters are determined by the total mass and density at their formation time. Under these assumptions, we calculate X-ray luminosity and temperature functions of galaxy clusters. We find that the behavior of luminosity function is different from the model which does not take account of formation epoch distribution, while the behavior of temperature function is not much changed. In our model, luminosity function is shifted to a higher luminosity and shows no significant evolution up to $z \sim 1$, independent of cosmological models. The clusters are populated on the temperature-luminosity plane with a finite dispersion. Since the simple scaling model in which the gas temperature is equal to the virial temperature fails to reproduce the observed luminosity-temperature relation, we also consider a model which takes the effects of preheating into account. The preheating model reproduces the observations much better.