An X-ray Study Of Galaxy Cluster Mergers And Their Effects On The ICM Chemical Evolution

Research around galaxy clusters,the largest known gravitationally bound systems in the Universe and the densest parts in the large-scale structure,is at the cross-roads of cosmology and astrophysics with great significance.Galaxy clusters are X-ray luminous sources due to high temperature(～0.5-10 keV)and high density(10-4-10-2 cm-2)of the intra-cluster medium(ICM).High-quality images and spectra obtained from Chandra and XMM-Newton X-ray observatories in combination with observations at other wavelengths(especially optical and radio)allowed for research of various interesting subjects in the galaxy clusters,such as the formation of cold fronts and shock fronts,ICM chemical evolution,gas cooling and heating,formation and evolution of diffuse radio sources,and the magnetic field distributions,which are all involved in cluster mergers.In this work,we conduct a detailed study on cluster merger dynamics through two examples:(1)galaxy group HCG 62 which possesses a highiron-abundance arc(～1 Z⊙)of unclear origin,and(2)galaxy cluster Abell 1775 which was reported to contain two interacting subclusters in the optical band and host one wideangle tail(WAT)and one narrow-angle tail(NAT)radio sources simultaneously in the core region(<40 kpc),indicating a complex merger dynamics.By analyzing Chandra and XMMNewton observations and performing smoothed particle hydrodynamic simulations,we aim to(1)illustrate the formation and evolution of X-ray gas substructure in merger events and understand whether they can be used to diagnose merger dynamics,and(2)study how mergers affect ICM chemical evolution.Our findings are as follows:(1)We find for HCG 62 that both iron and magnesium abundances form a high-abundance plateau broader than the previously reported high-abundance arc.We speculate this plateau to be formed by a recent merger event which may trigger starbursts contributing to additional enrichment and metal redistribution.We verify this conjecture through a series of hydrodynamical simulations using GADGET-3 code to reproduce the observed abundance distribution.The results demonstrate that HCG 62 is undergoing a major merger with a mass ratio of 3,and the merger-induced starbursts can be responsible for the high abundance plateau.(2)Our analysis on Abell 1775 X-ray observations identifies a head-tail substructure including an arc-shaped cold front and a cold stripped tail,and a plume of X-ray emission excess with spiral pattern.Along with optical and radio evidence and hydrodynamic simulations,we propose a two-body merger scenario where Abell 1775 is the primary cluster undergoing gas sloshing.The NAT radio galaxy is likely to be a single galaxy falling into the cluster center at a relative velocity of 2800 km s-1.We calculate the simulated turbulent velocity to be～570 km s-1 in the central region,which is consistent with the estimation from the radio minihalo with LOFAR observations,supporting the hypothesis of mini-halo’s origin that turbulence caused by gas sloshing re-accelerates primary electrons.Through a detailed study of cluster mergers in HCG 62 and Abell 1775,we conclude that(1)X-ray gas substructures can help diagnose the merger stage effectively,and(2)cluster mergers have significant effects on metal enrichment and redistribution.Meanwhile,this work provides insights into merger dynamics for our subsequent research on diffuse radio emissions,which causes noticeable foreground contamination in detecting epoch of reionization(EoR)HI signal.