title: The Growth of Brightest Cluster Galaxies and Intracluster Light in X-ray Selected Clusters
Creators: name: Furnell, K
Publisher: name: Liverpool John Moores University
Year: 2021

description: In this work, we examine the properties with respect to environment of brightest cluster galaxies (BCGs), the most massive population of galaxies in the known Universe, and also, the link between these galaxies and the intracluster light component (ICL) within galaxy clusters. The ICL is a flat, diffuse, low surface brightness (LSB) feature thought to contain a significant fraction of stellar mass contained within galaxy clusters; it is believed primarily to be formed from stripped stars during galaxy-galaxy interactions in the cluster core. Through our work, we attempt to discern the well-known observational and theoretical tensions which exist regarding the accretion history of stellar mass in the cores of galaxy clusters, as well as attempting to address some of the commonly-known systematics that accompany such studies. In Part I (Chapters 2-3), we analyse the structural parameters of a sample of 329 optically-confirmed, X-ray selected clusters in the SPectroscopic IDentification of eROSITA Sources Survey (SPIDERS) out to z ~ 0.3 with respect to three environmental properties (velocity dispersion-based halo mass, X-ray luminosity and optical richness). We quantify the morphologies of our BCGs by modelling their light profiles using GALFIT 3 (via the SIGMA pipeline), which we also use to obtain integrated magnitude-based stellar masses. We test the software thoroughly using ~ 20,000 mock galaxies implanted across 8 arbitrary cluster fields within our sample, which include field PSF convolutions and an idealised Poisson noise component. We find that we are able to recover our SDSS r-band profiles (with similar results in i and g) accurately out to ~ 16.5 magnitudes; fainter than this, we begin to observe biases in our results, especially in recovered Sersic index; such surface brightness-dependent structural biases have important consequences for evolutionary claims regarding BCG structure. Regarding environment, we find no evidence for any correlations between the structural properties of our science sample of 198 BCGs and any of the environmental properties used here, beyond BCG stellar mass. This observed homogeneity leads us to conclude that the majority of structural evolution of BCGs is likely to have occurred at earlier cosmic epochs than z ~ 0.3, with little evidence for much evolutionary activity at more recent epochs. In Part II, (Chapters 4-5), we attempt to quantify the ICL within a sample of 19 clusters detected in XMM Cluster Survey (XCS) data, with optical counterparts in the SDSS footprint. To do so, we use deep (~ 26.8) Hyper Suprime Cam Subaru Strategic Program DR1 (HSC-SSP DR1) i-band data. We apply a rest-frame mu_B = 25 mag/arcsec^2 isophotal threshold to our clusters, below which we define light as the ICL within an aperture of R_X,500 centered on the BCG. We apply extensive masking and careful post-processing correction techniques to address the persistent challenges which face observers attempting to analyse ICL; through application of our post-processing method, we recover a significant fraction (~ 1/5) of ICL flux previously lost during processing, significantly higher than our estimate of the image background at at the same isophotal level. We find, on average, that the ICL makes up about ~ 25% of the total cluster stellar mass on average (~ 41% including the flux contained in the BCG within 50kpc); this value is well-matched with other observational studies and SAM/numerical simulations, but is significantly smaller than results from recent hydrodynamical simulations (even when measured in an observationally consistent way). We find no evidence for any links between the amount of ICL flux with cluster mass, but find a growth rate of 2-4 for the ICL between 0 &lt; z &lt; 0.5; however, we find no evidence for any growth in our BCGs, finding the fractional contribution of BCG light to be strongly anti-correlated with halo mass at fixed redshift. We conclude that the ICL is the dominant evolutionary component of stellar mass in clusters from z ~ 1.