A Study Of X-ray Substructures And Central Gas Entropy Excess In Galaxy Groups And Clusters

Clusters of galaxies are the largest gravitationally bound systems in the Uni-verse. The hot gas (the intracluster medium; ICM) bounded in galaxy clustersradiates di?use emission in X-ray band, and is the largest mass component thatcan be seen and measured today. During the past ten years, many extraordinaryworks have been done based on the observations of Chandra and XMM-NewtonX-ray observatories, both of which have high spacial resolution and high energyresolution. In some merging galaxy clusters, researchers have found remarkableX-ray substructures, which often accompanied by shocks and/or cold fronts thatexhibit arc-shaped or edge-like morphologies. In some cool core clusters, re-searchers have found central X-ray cavities that are produced by AGN activity.These substructures are expected to contain valuable information not only onmerger dynamics itself, but also on thermal and chemical evolutions of the ICM,which helps study structure formation and evolution and understand the natureof both dark matter and central supermassive black holes. In the thesis, we focuson the origin of X-ray substructures in merging clusters and central gas entropyexcess associated with AGN feedback to heat its surrounding gas.By analyzing the Chandra and XMM-Newton archived data of the nearbygalaxy cluster Abell 3158, which was reported to possess a relatively regular,relaxed morphology in the X-ray band in previous works. We identify a bowedge-shaped discontinuity in the X-ray surface brightness distribution at about120h?711 kpc west of the X-ray peak. This feature is found to be associated witha massive, o?-center cool gas clump, and actually forms the west boundary ofthe cool clump. By calculating the thermal gas pressures in the cool clump andin the free-stream region, we determine that the cool gas clump is moving ata subsonic velocity toward west on the sky plane. We exclude the possibility that this cool clump was formed by local inhomogeneous radiative cooling in theICM, due to the e?ectiveness of the thermal conduction on the time-scale of～0.3Gyr. Since no evidence for central AGN activity has been found in Abell 3158,and this cool clump bears many similarities to the o?-center cool gas clumpsdetected in other merging clusters in terms of their mass, size, location, andthermal properties, we speculate that the cool clump in Abell 3158 was causedby a merger event, and is the remnant of the original central cool-core of the maincluster or the infalling sub-cluster. This idea is supported not only by the studyof line-of-sight velocity distribution of the cluster member galaxies, but also bythe study of gas entropy-temperature correlation. This example shows that theappearance of such massive, o?-center cool gas clumps can be used to diagnosethe dynamical state of a cluster, especially when apparent merger signatures (e.g.distinct shocks and cold fronts) have vanished in the late merger stage.By analyzing Chandra X-ray data of a sample of 40 galaxy groups andclusters, we ?nd that in 31 sample systems there exists a signi?cant central gasentropy excess, which corresponds to ? 0.1?0.5 keV/gas particle. We also ?nd adistinct correlation between the central entropy excess and K-band luminosity ofthe central dominating galaxies (CDGs), which is scaled as ?K0∝L1K.6±0.4, whereLK is tightly associated with the mass of the supermassive black hole hosted inthe CDG. In fact, if an e?ective mass-to-energy conversion-e?ciency of 0.02 isassumed for the accretion process, the cumulative AGN feedback yields an extraheating of ? 0.5?17.0 keV/gas particle, which is su?cient to explain the centralentropy excess. In most cases the AGN contribution can compensate the radiativeloss of the X-ray gas within the cooling radius (? 0.002 ? 2.2 keV/gas particle),and apparently exceeds the energy required to deviate the scaling relations fromthe self-similar predictions (? 0.2 ? 1.0 keV/gas particle). In contrast to theAGN feedback, the extra heating provided by supernova explosions accounts for? 0.002 ? 0.08 keV/gas particle in groups and is almost negligible in clusters.Therefore, the observed correlation ?K0 ? LK can be considered as a directevidence for AGN feedback in galaxy groups and clusters.