Inside a Beehive: The Multiple Merging Processes in the Galaxy Cluster Abell 2142

Abstract

To investigate the dynamics of the galaxy cluster A2142, we compile an extended catalog of 2239 spectroscopic redshifts of sources, including 237 newly measured redshifts, within 30 arcmin from the cluster center. With the σ-plateau algorithm from the caustic method, we identify 868 members and a number of substructures in the galaxy distribution both in the outskirts, out to ∼3.5 Mpc from the cluster center, and in the central region. In the outskirts, one substructure overlaps a falling clump of gas previously identified in the X-ray band. These substructures suggest the presence of multiple minor mergers, which are responsible for the complex dynamics of A2142, and the absence of recent or ongoing major mergers. We show that the distribution of the galaxies in the cluster core and in several substructures is consistent with the mass distribution inferred from the weak-lensing signal. Moreover, we use spatially resolved X-ray spectroscopy to measure the redshift of different regions of the intracluster medium within ∼3 arcmin from the cluster center. We find a ring of gas near the two X-ray cold fronts identified in previous analyses and measure a velocity of this ring of 810 ± 330 km s<SUP>-1</SUP> larger than the cluster mean velocity. Our analysis suggests the presence of another ring surrounding the core, whose velocity is 660 ± 300 km s<SUP>-1</SUP> larger than the cluster velocity. These X-ray features are not associated with any optical substructures, and support the core-sloshing scenario suggested in previous work. <P />We dedicate this paper to the late Bepi Tormen, our beloved friend and colleague whose enthusiastic and intense work on gravitational dynamics largely contributed to our current understanding of the formation and evolution of galaxy clusters.