A Study of Brightest Cluster Galaxies Over the Last Five Billion Years

Abstract

The location of Brightest Cluster Galaxies (BCGs), at the centre of the clusters of galaxies, suggests a strong link between the formation and evolution of the BCG and that of the host cluster; with their unique environment distinguishing BCG evolution from that of massive field ellipticals. In this thesis I explore some of the contributing factors to the unique evolution of BCGs. In particular, I investigate the interaction of gas cooling from the intracluster medium (ICM) and the BCG, in fuelling star formation and AGN activity in BCGs, as well as the properties of the multi-phase gas itself. I also investigate the stellar mass growth of BCGs through galaxy--galaxy mergers.

I attempt to constrain the prevalence of BCGs undergoing star formation and/or AGN activity by conducting a multi-wavelength photometric census of 981 BCGs, from an X-ray selected sample of clusters at z<0.5. It is found that at least 14 per cent of the BCGs have colours which deviate from those expected for a passively evolving galaxy. A strong association is discovered between BCG colours and optical line emission, indicating the presence of multi-phase gas and a strong association is also discovered between BCG colour and the X-ray luminosity of the host cluster, with implications regarding the proportion of active BCGs and the selection of a particular sample of clusters. These results demonstrate that a significant fraction of BCGs are in fact active, contrary to their ``red and dead'' reputation.

The theme of AGN activity in BCGs is continued via an investigation searching for clusters around some 3500 ROSAT selected AGN. The aim is to address the ambiguity that exists between clusters and AGN in low resolution X-ray imaging, which disfavours the detection of the inherently rare systems where a strong AGN is hosted by a BCG. I identify 22 candidate systems with significant overdensities of red galaxies, with several being independent rediscoveries of such systems. Six best candidate systems are found where the cluster and AGN X-ray emission are likely to be comparable. Identification of such systems will ultimately aid in our understanding of the role of AGN feedback in this unique environment.

The stellar mass assembly of BCGs at z<0.25 is also explored, targeting 23 merger--like BCGs, drawn from a large parent cluster sample, with wide-field integral field spectroscopy. The stellar kinematics are used to determine a bound probability for companion cluster galaxies and average merger timescales are estimated. Due to selection effects the average BCG stellar mass growth cannot be constrained tighter than between 9(+/-3) and 57(+20/-17) per cent, with typical values in the literature of 10-20 per cent. Nevertheless, the techniques utilised in this analysis should help lay the groundwork for subsequent studies; demonstrating the power of using a statistically significant number of IFU observations to study BCG mergers.

Spatially resolved optical line emission is also explored in ten cluster cores. A variety of morphologies and kinematic profiles are discovered, and the properties of each individual system are discussed in turn. The most striking results relate to two systems where the ionised gas lies offset from the BCG. An X-ray observation of A2566 confirms the X-ray peak is offset from the BCG and coincident with the offset line emission, which, along with previous observations, suggest cooling of the ICM can occur in isolation of, and away from, the BCG -- likely related to sloshing of the ICM. A 40 kpc filament is discovered in A2533, offset from the BCG and spatially coincident with star formation and dust lane features in the continuum. An ALMA observation also finds cold molecular gas, which almost perfectly traces the warm ionised gas. Without a high resolution X-ray observation a definitive link between the offset emission and the ICM cannot be established for this system, however, we predict the X-ray peak will be offset, analogous to A2566. Such systems allow the rare opportunity of studying cooling flows in isolation of the role of AGN feedback within BCGs.