Evolution of the Brightest Cluster Galaxies

Abstract

In this thesis, I explore the evolution of the brightest cluster galaxies (BCGs) over the last 10 billion years through detailed studies of both local BCGs from SDSS and their high-z progenitors from CANDELS UDS. First, I study a large sample of local BCGs and link their morphologies to their structural properties. We derive visual morphologies for these BCGs and find that ~57% of local BCGs are cD galaxies, ~13% are ellipticals, and ~21% belong to intermediate classes, mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (elliptical BCGs) to clearly detected (cD galaxies), with intermediate classes showing increasing degrees of the envelope presence. A minority (~7%) of BCGs have disk morphologies, with spirals and S0s in similar proportions, and the rest (~2%) are mergers. After carefully fitting the galaxy light distributions using Sersic models, I find a clear link between BCG morphology and structure, such that cD galaxies are typically larger than elliptical BCGs, and the visually extended envelope of cD galaxies is a distinct structure differing from the central bulge. Based on this BCG morphology--structure correlation, I develop a statistically robust way to separate cD from non-cD BCGs, by which cD galaxies can be selected with reasonably high completeness and low contamination. Next, I investigate the effect of environment on the properties of local BCGs by studying the relationship between the BCG's internal properties (stellar mass, structure and morphology) and their environment (local density and cluster halo mass). I find that the size of BCGs is determined by the intrinsic BCG stellar mass, with a weak correlation with the cluster environment. Additionally, more massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. The growth of the BCGs seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. Moreover, I demonstrate that cD galaxies are ~40% more massive than elliptical BCGs, and prefer denser regions and more massive haloes. My results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. I propose that most present-day cDs started their life as ellipticals at z~1, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. This process is nearing completion since the majority of the local BCGs have cD morphology. However, the presence of BCGs with intermediate morphological classes suggests that the growth and morphological transformation of BCGs is still ongoing. Finally, I present a new method for tracing the evolution of BCGs from z~2 to z~0. I conclude, on the basis of semi-analytical models, that the best method to select BCG progenitors at z~2 is a hybrid environmental density and stellar mass ranking approach. Ultimately I am able to retrieve 45% of BCG progenitors. Although the selected high-z progenitor sample is a mixture of BCG and non-BCG progenitors, I demonstrate that their properties can be used to trace BCG evolution. Applying this method to the CANDELS UDS data, I construct an observational BCG progenitor sample at z~2. A local BCG comparison sample is constructed using the SDSS data, taking into account the likely contamination from non-BCGs to ensure a fair comparison between high-z and low-z samples. Using these samples I demonstrate that BCG sizes have grown by a factor of ~3.2 since z~2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early-type local counterparts. I also find that BCG progenitors have more disturbed morphologies, while local BCGs have much smoother profiles. Moreover, I find that the stellar masses of BCGs have grown by a factor of ~2.5 since z~2, and the SFR of BCG progenitors has a median value of ~14 Msun/yr, much higher than their quiescent local descendants. I demonstrate that at 1<z<2 star formation and merging contribute approximately equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z<1. I also find that BCG progenitors at high-z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z<2