Investigating the build-up of brightest cluster galaxies over half of cosmic time

Abstract

Central galaxies in galaxy clusters are special. These galaxies referred to in the literature as brightest cluster galaxies (BCGs), are the most luminous (or massive) galaxies in their host clusters and are located at the bottom of the clusters' gravitational potential wells. The stellar mass growth of these massive galaxies is a hotly debated topic. The general picture emerging from simulations suggests that star formation dominates the early stellar mass assembly of the BCGs (at z ≥ 2), while dry mergers are the dominant mechanism through which these massive galaxies increase their stellar masses at later times (z ≤ 1). The simulations however predict a much higher mass growth for the BCGs than is observed. Observational studies on the other hand report conflicting results regarding the stellar mass growth of BCGs since z ≤ 1. Some studies find that the BCGs have not experienced any mass growth since z ~ 1, while others measure significant growth over the same redshift range. In this thesis we investigate the importance of major mergers in the stellar mass build-up of BCGs between 0.08 ≤ z ≤ 0.50. We do this using two independent methods where we respectively measure the: (i) total and (ii) merger-inferred stellar mass growth of the BCGs. In the first method we simply investigate how the average stellar masses of the BCGs change as a function of redshift. In this analysis we account for the relation between the BCG's stellar mass and the host cluster's halo mass in selecting the BCG sample. In the second method we use close pair statistics as a diagnostic of mergers. Thereafter the close pair fraction is used as a proxy for the BCG merger fraction in order to estimate how much stellar mass growth the BCGs will experience due to mergers. We find that major mergers will contribute 24±14% on average towards the stellar mass growth of a present day BCG since z = 0.32. With regards to the total stellar mass growth of the BCGs, we find that it is 0+15%-3% between 0.08 ≤ z ≤ 0:35. The upper limit on this result depends sensitively on the correction applied for the SDSS photometry of the brightest, largest (lowest redshift) galaxies. The total mass growth is lower than the merger-inferred growth, although they are consistent within the uncertainties. We find that it is important to take the growth of the clusters' halo masses into account, otherwise one runs the risk of obtaining the non-physical result that BCGs are somehow losing stellar mass towards the present day (i.e. shrinking). We further find that the stellar mass growth of BCGs appears to be independent of the range of parent halo masses probed (providing these clusters are descendants / progenitors of each other). We also find that major mergers are the dominant contributor to the intracluster light stellar mass build-up at low redshifts. Together the total and merger-inferred stellar mass growth results imply that mergers are the dominant mechanism through which a BCG's stellar mass can grow at low redshift.