Brightest cluster galaxies: optical properties at intermediate redshift




Bildfell, Christopher John

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Galaxies grow and evolve via the repeated process of hierarchical merging, with smaller galaxies being cannibalized by larger and correspondingly brighter ones. Thus in galaxy clusters it is the brightest cluster galaxy (BCG) that lies at the top of the galaxian food chain and deep within the cluster potential the BCG grows to become one of the most massive galaxies in the universe. Baryonic feedback processes associated with the formation of the BCG (eg., AGN, star formation, stellar winds and chemical enrichment) affect the balance between heating and cooling of the intra-cluster medium (ICM) which in turn, through replenishment of the cold gas reservoir of the BCG, feeds back on these processes. This interconnection between the BCG and the state of the cluster gas has direct implications for theories of both galaxy and cluster evolution. Thanks to recent advances in observational techniques it has become possible to study the link between BCG and host cluster properties across a wide range of multi-wavelength information. Using deep imaging data taken with the Canada-France-Hawaii Telescope as part of the Canadian Cluster Comparison Project. (CCCP). we examine the surface brightness distributions in g' and r' of 28 BCGs in a redshift range 0.15 z 0.55 residing in massive clusters with Tx > 5 and compare them to the global X-ray properties of their hosts. We fit R1/4 models to the surface brightness profiles and find that the Kormendy relation of BCGs, when corrected for passive evolution. is consistent with that of the local elliptical population. We identify a subset of BCGs with extraordinarily blue centers (0.1 < dlog(g' - r')/dlog(r) < 1.3 within 15 h70-1 kpc) extending several times the fwhm of the psf. These objects all lie on the most luminous side of the scatter in the Lx-Tx relation for a given T1, an effect which previous authors have shown can be explained by cooling of the ICM (McCarthy et al. 2004). We find independent evidence from previous studies that these systems are all hosts of recent star formation. The blue cores in these BCGs cause them to be displaced from the red sequences of their host clusters; in the most extreme cases this displacement is 11-nag. These findings are contradictory to the current paradigm of elliptical galaxy formation within clusters where star formation is thought to have a negligible effect in the most massive cluster ellipticals at late cosmic times. Furthermore BCG total magnitude is found to correlate with host cluster deviation from the mean Lx-Tx relation. We argue that cooling and pre-heating of the Intra-Cluster-Medium has an important affect on the global and structural properties of the stellar component of BCGs. Not all BCGs are red and dead, there is a need to incorporate an external source of cold gas replenishment into quenching models of galaxy formation in clusters in order to reconcile these observations.



galaxies, clusters, red shift