Detailed X-ray properties of galaxy groups and fossil groups

Abstract

Most galaxies in the Universe are aggregated into groups of galaxies, agglomerations of a few 10s of galaxies at most. For long they have been considered to be very similar to clusters, which contain a few 100s of galaxies. Recent scientific developments however suggest that the two types of systems do not have the exact same properties. In this dissertation, the goal was to study the similarities and differences between groups and clusters, for a selection of properties, primarily of the hot X-ray emitting gas, i.e. the intracluster medium. This was carried out via three sub-projects.<br /> In the first project, the goal was to investigate the cool-core properties of a sample of 26 galaxy groups with Chandra data and correlate it to the feedback from the supermassive black hole (SMBH) in the group centres. This involved handling data in three wavelengths, namely, X-ray, radio, and near-infrared (NIR). For the X-ray analysis, the Chandra data was used to extract temperature and density profiles and constrain the central cooling time (CCT) of the gas. The CCT was used to classify the galaxy groups into strong cool-core, weak cool-core, and non cool-core classes, which was done for the first time for an objectively selected galaxy group sample. The radio output of the central active galactic nucleus (AGN) was constrained using catalogue data and correlated to the CCT. The mass of the central SMBH was determined using NIR data for the brightest cluster galaxy (BCG) from the 2MASS extended source catalogue (XSC), and a scaling relation. Finally, the scaling relation between the X-ray luminosity/mass of the galaxy group and cluster (L_X-M500) and the NIR luminosity of the BCG (LBCG) was extended all the way from the cluster regime to the group regime. The results show that although the observed cool-core fraction is similar in galaxy groups and clusters, there are important differences between the two classes of objects. Firstly, despite having very short CCTs (CCT < 1 Gyr), there are some galaxy groups which have a centrally rising temperature profile unlike what is observed for clusters. Secondly, there is an absence of a correlation between the CCT and the central radio-loud AGN fraction in groups unlike that for clusters. Thirdly, the indications of an anti-correlation trend between the CCT and the radio luminosity of the central AGN observed for clusters is not seen for galaxy groups. Fourthly, the weak correlation between the radio luminosity and the mass of the SMBH observed for strong cool-core (SCC) galaxy clusters is absent for SCC galaxy groups. Finally, the strong correlation for the L_X -LBCG and the M500-LBCG scaling relation observed for clusters weakens significantly when the scaling relation is extended to the group regime. In the second project, the bolometric luminosity (L_X)-temperature (T) scaling relation was extended from the cluster regime to the group regime. Additionally, we studied the impact of ICM cooling and AGN feedback on the scaling relation for the first time for galaxy groups by fitting the relation for individual sub-samples, accounting for different cases of ICM cooling and AGN feedback. The impact of selection effects were qualitatively and quantitatively examined using simulations, and bias-corrected relations were established for the entire sample and all sub-samples. The slope of the bias-corrected L_X-T relation is marginally steeper but consistent within errors to that of clusters (~ 3), with the relation being steepest and highest in normalisation for the strong cool-core groups (CCT < 1 Gyr), and shallowest for those groups without a strong cool-core. The statistical scatter in T on the group regime is comparable to the cluster regime, while the statistical and intrinsic scatter in L_X increases. We report for the first time that the bias-corrected intrinsic scatter in L_X is higher than the observed scatter for groups. We also see indications that groups with a relatively powerful radio-loud AGN have a much steeper L_X-T relation. Finally, we speculate that such powerful radio AGN are preferentially located in groups which lack a strong cool-core.<br /> The scientific goal of the third project was to investigate the core properties of fossil systems, an interesting sub-class of clusters/groups in detail for the very first time using Chandra archival data for 17 systems. The presence/absence of a cool-core in fossils was determined via three diagnostics, namely the CCT, Cuspiness, and concentration parameter. The X-ray peak/BCG separation and the X-ray peak/emission weighted centre separation was quantified to give an indication of the dynamical state of the system. We also studied five low redshift fossils (z < 0.05) in detail, and obtained their deprojected ICM properties. Lastly, we also studied the L_X-T relation for fossils from the 400d catalogue which shows indications of being higher in normalisation compared to other galaxy groups, after factoring in cool-core nature and potential selection effects. We finally interpreted these results within the context of the formation and evolution of fossils.