On the relation between the X-ray properties of clusters of galaxies and their brightest cluster member

Abstract

The recently established correlations between the absolute magnitude of the Brightest Cluster Member (BCM) and the X-ray properties of its host cluster are used to refine its use as a standard candle and provide new insights into BCM formation and evolution. The correction for cluster temperature reduces the scatter in the Hubble diagram significantly. The implications for this method are discussed. The optical-near-infrared colour ratios of the BCMs are found to be weakly correlated with the X-ray properties, indicating that BCMs in more massive clusters tend to be bluer. The optical properties of BCMs are not directly related to the size of any cooling flow, indicating that the formation of stars from gas condensing out of the flow is biased heavily toward stars with masses less than 0.5 M. The close relationship between cluster and BCM properties indicates that BCM formation and evolution are dictated by the cluster, through a combination of mergers of galaxies at cluster collapse, accretion of a 'dark', baryonic halo through a cooling flow, and ongoing mergers with other BCMs from infalling subclusters.