A 2163: Merger events in the hottest Abell galaxy cluster

Abstract

Context. A 2163 is among the richest and most distant Abell clusters, presenting outstanding properties in different wavelength domains. X-ray observations have revealed a distorted gas morphology and strong features have been detected in the temperature map, suggesting that merging processes are important in this cluster. However, the merging scenario is not yet well-defined. Aims. We have undertaken a complementary optical analysis, aiming to understand the dynamics of the system, to constrain the merging scenario and to test its effect on the properties of galaxies. Methods. We present a detailed optical analysis of A 2163 based on new multicolor wide-field imaging and medium-to-high resolution spectroscopy of several hundred galaxies. Results. The projected galaxy density distribution shows strong subclustering with two dominant structures: a main central component (A), and a northern component (B), visible both in optical and in X-ray, with two other substructures detected at high significance in the optical. At magnitudes fainter than R = 19, the galaxy distribution shows a clear elongation approximately with the east-west axis extending over 4 h −1 70 Mpc, while a nearly perpendicular bridge of galaxies along the north-south axis appears to connect (B) to (A). The (A) component shows a bimodal morphology, and the positions of its two density peaks depend on galaxy luminosity: at magnitudes fainter than R = 19, the axis joining the peaks shows a counterclockwise rotation (from NE/SW to E-W) centered on the position of the X-ray maximum. Our final spectroscopic catalog of 512 objects includes 476 new galaxy redshifts. We have identified 361 galaxies as cluster members; among them, 326 have high precision redshift measurements, which allow us to perform a detailed dynamical analysis of unprecedented accuracy. The cluster mean redshift and velocity dispersion are respectively z = 0.2005 ± 0.0003 and 1434 ± 60 km s −1 . We spectroscopically confirm that the northern and western components (A 2163-B and A 2163-C) belong to the A 2163 complex. The velocity distribution shows multi-modality, with an overall bimodal structure peaking at ∼59 200 km s −1 and ∼60 500 km s −1 . A significant velocity gradient (∼1250 km s −1 ) is detected along the NE/SW axis of the cluster, which partially explains the detected bimodality. A 2163 appears to be exceptionally massive: the cluster virial mass is M vir = 3.8± 0.4× 10 15 M h −1 70 . Conclusions. Our analysis of the optical data, combined with the available information from X-ray observations and predictions of numerical simulations, supports a scenario in which A 2163-A has undergone a recent (t ∼ 0.5 Gyr) merger along a NE/SW (or E-W) axis, and A 2163-B is connected to the main complex, and is probably infalling on A 2163-A.

Merging history of three bimodal clusters

“…Moreover, we have also shown that in recent post-merger clusters, such as A2440 (this work) and A2163 (Maurogordato et al 2008), galaxies exhibit a strong luminosity segregation, similar to the case of the Coma cluster studied by Biviano et al (1996). We have also found a deviation from the σ − T X relation for the subclusters in a post-merger stage, the largest one (at the 3σ level) being detected for A2384N.…”

A2163: Merger events in the hottest Abell galaxy cluster

“…1 10 38 W ≡ 10 45 erg s −1 ; X-ray luminosity in the 2-10 keV band has been corrected for luminosity distance assuming H 0 = 70 km s −1 Mpc −1 , and Λ = 0.7. Squires et al 1997;Radovich et al 2008) and in the galaxy density distribution (Maurogordato et al 2008). Moreover, A2163 is known to host a prominent radio halo (Feretti et al 2001(Feretti et al , 2004 and to be a possible source of non thermal hard X-ray emission, as suggested by Rephaeli et al (2006) from RXTE observations and Million & Allen (2009) from Chandra data analysis.…”

The effect of ram pressure on the star formation, mass distribution and morphology of galaxies

“…From optical spectroscopical investigations it is evident that galaxies have velocities of up to several 1000 km s −1 in the centre of galaxy clusters, depending on the dynamical state of the galaxy cluster and the total mass of the system, e.g. Maurogordato et al (2008). We split our simulation into three major groups, first the fast moving galaxies with a relative velocity of 1000 km s −1 , second an intermediate relative velocity of 500 km s −1 and third a group of very slow moving galaxies with a relative velocity of 100 km s −1 with respect to the ambient medium.…”