The evolution of galaxies in proto-clusters

In this thesis, I report on my work on galaxy evolution in proto-clusters. In Chapter 1, I describe previous knowledge about clusters and proto-clusters of galaxies. The next three Chapters correspond to three different projects I have completed during my PhD that are connected to the topic of galaxy evolution in proto-clusters. In Chapter 2, I quantify the relative importance of environmental quenching versus pre-processing in z~1 clusters by analysing the infalling galaxy population in the outskirts of 15 galaxy clusters at 0.8 < z < 1.4 drawn from the GOGREEN and GCLASS surveys. I find significant differences between the infalling galaxies and a control sample; in particular, an excess of massive quiescent galaxies in the infalling region. These massive infalling galaxies likely reside in larger dark matter haloes than similar-mass control galaxies because they have twice as many satellite galaxies. Based on these findings, I conclude that it may not be appropriate to use 'field' galaxies as a substitute for infalling pre-cluster galaxies when calculating the efficiency and mass dependency of environmental quenching in high redshift clusters. By comparing the quiescent fraction of infalling galaxies at 1 < R/Rv < 3 to the cluster sample (R/Rv < 1) I find that almost all quiescent galaxies with masses >10¹¹ M☉ were quenched prior to infall, whilst up to half of lower mass galaxies were environmentally quenched after passing the virial radius. This means most of the massive quiescent galaxies in z~1 clusters were self-quenched or pre-processed prior to infall. In Chapter 3, I report on my work on intracluster light in proto-clusters at z~2. In contrast to theoretical expectations, I report on the detection of intracluster light within two proto-clusters at z=2 using deep HST images. I use the colour of the intracluster light to estimate its mass-to-light ratio in annuli around the brightest cluster galaxies (BCG), up to a radius of 100 kpc. I find that 54±5% and 71±3% of the stellar mass in these regions is located more than 10 kpc away from the BCGs in the two proto-clusters. This low concentration is similar to BCGs in lower redshift clusters, and distinct from other massive proto-cluster galaxies. This suggests that the proto-cluster BCGs have already experienced a special merger history similar to their lower redshift counterparts. We compare these observations to the Hydrangea hydrodynamical galaxy cluster simulations. In contrast to semi-analytic models, they predict that intracluster stars are a generic feature of massive halos since at least z=2. This implies that intracluster light is a natural consequence of hierarchical structure formation. Over Chapter 4, I report on my work on the luminosity function and Dn4000 measurements of proto-cluster galaxies at 1.3 < z < 3.0. Using HST grism data, I found that the luminosity function of proto-clusters differ from the field, such that proto-clusters have an excess of luminous galaxies. I also found that proto-cluster galaxies have higher values of Dn4000 compared to field galaxies. We interpret that proto-cluster galaxies have older stellar populations than field galaxies. In Chapter 5, I summarise my main findings on this work and plans for future work.