Formation of bright central galaxies in massive haloes

Galaxy formation is one of the most active and evolving fields of research in observational astronomy and cosmology. While we know today which physical processes qualitatively regulate galaxy evolution, the precise timing and the behaviour of these processes and their relations to host environments remain unclear. Many interesting questions are still debated: What regulates galaxy evolution? When do massive galaxies assemble their stellar mass and how? Where does this mass assembly occur? This thesis studies the formation and evolution of central galaxies in groups and clusters over the last 9 billion years in an attempt to answer these questions.

Two important properties of galaxy clusters and groups make them ideal systems to study cosmic evolution. First, they are the largest structures in the Universe that have undergone gravitational relaxation and virial equilibrium. By comparing mass distributions among the nearby- and early-Universe clusters, we can measure the rate of the structure growth and formation. Second, the gravitational potential wells of clusters are deep enough that they retain all of the cluster material, despite outflows driven by supernovae (SNe) and active galactic nuclei (AGN). Thus, the cluster baryons can provide key information on the essential mechanisms related to galaxy formation, including star formation efficiency and the impact of AGN and SNe feedback on galaxy evolution. This thesis reports the identification of a large sample of galaxy groups including their optical and X-ray properties. It includes several refereed journal articles, of which five have been included here.

In the first article (Gozaliasl et al. 2014a), we study the distribution and the development of the magnitude gap between the brightest group galaxies and their brightest satellites in our well defined mass-selected sample of 129 X-ray galaxy groups at 0.04 < z < 1.23 in XMM-LSS. We investigate the relation between magnitude gap and absolute r-band magnitude of the central group galaxy and its brightest satellite. Our observational results are compared to the predictions by three semi-analytic models (SAMs) based on the Millennium simulation. We show that the fraction of galaxy groups with large magnitude gaps (e.g. fossils) increases significantly with decreasing redshift by a factor of ∼ 2. In contrast to the model predictions, we show that the intercept of the relation between the absolute magnitude of the brightest groups galaxies (BGGs) and the magnitude gap becomes brighter as a function of increasing redshift. We attribute this evolution to the presence of a younger population of the observed BGGs

In the second article (Gozaliasl et al. 2016), we study the distribution and evolution of the star formation rate (SFR) and the stellar mass of BGGs over the last 9 billion years, using a sample of 407 BGGs selected from X-ray galaxy groups at 0.04 < z < 1.3 in the XMM-LSS, COSMOS, and AEGIS fields. We find that the mean stellar mass of BGGs grows by a factor of 2 from z = 1.3 to present day and the stellar mass distribution evolves towards a normal distribution with cosmic time. We find that the BGGs are not completely inactive systems as the SFR of a considerable number of BGG ranges from 1 to 1000 M_sun/yr.

In the third article (Gozaliasl et al. 2014b), we study the evolution of halo mass, magnitude gap, and composite (stacked) luminosity function of galaxies in groups classified by the magnitude gap (as fossils, normal/non-fossils, and random groups) using the Guo et al. (2011) SAM. We find that galaxy groups with large magnitude gaps, i.e. fossils (∆M1,2 ≥ 2 mag), form earlier than the non-fossil systems. We measure the evolution of the Schechter function parameters, finding that M∗ for fossils grows by at least +1 mag in contrast to non-fossils, decreasing the number of massive galaxies with redshift. The faint-end slope (α) of both fossils and non-fossils remains constant with redshift. However, φ∗ grows significantly for both type of groups, changing the number of galaxies with cosmic time. We find that the number of dwarf galaxies in fossils shows no significant evolution in comparison to non-fossils and conclude that the changes in the number of galaxies (φ∗) of fossils are mainly due to the changes in the number of massive (M∗) galaxies. Overall, these results indicate that the giant central galaxies in fossils form by multiple mergers of the massive galaxies.

In the fourth article (Khosroshahi et al. 2014), we analyse the observed X-ray, optical, and spectroscopic data of four optically selected fossil groups at z ∼ 0.06 in 2dFGRS to examine the possibility that a galaxy group, which hosts a giant luminous elliptical galaxy with a large magnitude gap, can be associated with diffuse X-ray radiation, similar to that of fossil groups. The X-ray and optical properties of these groups indicate the presence of extended X-ray emission from the hot intra-group gas. We find that one of them is a fossil group, and the X-ray luminosity of two groups is close to the defined threshold for fossil groups. One of the groups is ruled out due to the optical contamination in the input sample.

In the fifth paper (Khosroshahi et al. 2015), we analyse data from the multiwavelength observations of galaxy groups to probe statistical predictions from the SAMs. We show that magnitude gap can be used as an observable parameter to study groups and to probe galaxy formation models.