The X-ray Study Of The Bright Central Galaxies And Hot Gas Of Galaxy Clusters

In the standard ACDM cosmological model,cosmic structure formation is hierarchical,i.e.,objects of small scales form first and then gradually grow into larger-scale structures upon mergers and accretion.Although the dark matter merger history is in good agreement with simulations,the current understanding of the baryonic evolution process is still relatively poor,which further hinders our understanding of galaxy formation and galaxy cluster evolution.On one hand,for galaxies,especially the very bright galaxies at the center of galaxy clusters(BCGs,typically more luminous than other member galaxies by a factor of 10,which harbors the most massive black holes in the Universe),there are still many hypotheses about their formation mechanisms,e.g.,the rapid cooling of the intracluster medium(ICM)gas,the cannibalization of surrounding small-mass galaxies by the central massive galaxy,the rapid merging of galaxies,and tidal disintegration.However,observations are still required to verify these formation hypotheses and to explain the details of the BCG formation.On the other hand,gas structures produced during the merging process of galaxy clusters,such as cold fronts and turbulence,can help us to understand the gas transport processes in galaxy clusters(e.g.,viscous force effects,heat conduction,and magnetic fields).But such studies call for high spatial resolution X-ray observations.Motivated by the above scientific topics,our collaborators have proposed Chandra observations,whose spatial resolution reaches 0.5",the highest among current X-ray instruments,and have acquired about 173-ks long first-hand data.The following topics have then been investigated based on this set of data:(1)X-ray study of the central bright galaxy of galaxy cluster Abell 407 and the AGN feedback.A407 contains a compact group of galaxies in the central 30 kpc of Abell 407(A407),in which 9 members galaxies are identified and also show merging features.By analyzing the cumulative 44 ks Chandra observations of galaxy cluster A407,in addition to radio and optical observations,we·verify the disruptive effect on galaxy hot gas corona by galaxy merger,and estimate the merger mass and timescale for future forming BCG;·determine the location of the host galaxy of the radio AGN,then measure the temperature distribution,the gas entropy distribution,and the cooling time distribution,of the intracluster medium(ICM),and estimate the energy of AGN bursts in galaxy clusters.We conclude that the merger of galaxies is a feasible mechanism of BCG formation,and that the AGN feedback affects both the kinetic and thermodynamic properties of the ICM,such as heating of the ICM gas,and enriching the metal abundances.(2)Study of physical processes in the hot gas of the ICM in galaxy cluster MCXC J0157.4-0550.The galaxy cluster SDSS J015733(J0157 for short),as a subcluster,is currently merging with its main cluster and producing the brightest sloshing-type cold front structure observed so far,and the cold front scale is one of the largest ever found(～260 kpc).By analyzing the cumulative Chandra X-ray observations of 129 ks for galaxy cluster J0157,we found that·The high spatial resolution observations of Chandra indicate that the KH instability of the cold front is suppressed;·Accordingly,we also give lower bound estimates for the viscous and magnetic fields,respectively.Accordingly,we conclude that the merger activity of galaxy clusters produces large-scale cold front structures;and that the KH instability is strongly suppressed by the ICM magnetic field and gas viscous forces,and that the cold fronts can remain sharp on large scales;The results also suggest that the magnetic field and gas viscosity in galaxy clusters can have very important effects on the dynamical and thermodynamic state of the ICM,as well as on the cosmology and baryon evolution in galaxy clusters.Our collaborators searched through SDSS data and found multiple multicore systems similar to the A407.We will further investigate the evolution of complex multinuclear systems in the Universe by searching for more cases of low and high redshift BCGs.For the study of the dynamical and thermodynamic properties of the hot gas in galaxy clusters,we will further explore the work of numerical simulations to make a more detailed distinction between the role of viscous and magnetic fields in the ICM.These works are indispensable and crucial to our current understanding of galaxy and galaxy cluster formation,and the evolution of cosmic baryonic matter.