Abell 851 and the Role of Starbursts in Cluster Galaxy Evolution

TL;DR

This study investigates the origins and characteristics of starburst galaxies in the z ~ 0.4 galaxy cluster Abell 851, revealing their association with infalling groups and the cluster core, and their role in galaxy evolution.

Contribution

It provides new observational evidence linking starbursts to specific environments within the cluster and discusses their implications for galaxy transformation processes.

Findings

Starbursts are mainly found in infalling groups and the cluster center.

Major mergers at the center lead to dust-enshrouded starbursts and proto-S0 formation.

Starbursts in infalling groups are less often linked to major mergers.

Abstract

We use extensive new observations of the very rich z ~ 0.4 cluster of galaxies A851 to examine the nature and origin of starburst galaxies in intermediate-redshift clusters. New HST observations, Spitzer photometry and ground-based spectroscopy cover most of a region of the cluster about 10 arcmin across, corresponding to a clustercentric radial distance of about 1.6 Mpc. This spatial coverage allows us to confirm the existence of a morphology-density relation within this cluster, and to identify several large, presumably infalling, subsystems. We confirm our previous conclusion that a very large fraction of the starforming galaxies in A851 have recently undergone starbursts. We argue that starbursts are mostly confined to two kinds of sites: infalling groups and the cluster center. At the cluster center it appears that infalling galaxies are undergoing major mergers, resulting in starbursts whose optical emission lines are completely buried beneath dust. The aftermath of this process appears to be proto-S0 galaxies devoid of star formation. In contrast, major mergers do not appear to be the cause of most of the starbursts in infalling groups, and fewer of these events result in the transformation of the galaxy into an S0. Some recent theoretical work provides possible explanations for these two distinct processes, but it is not clear whether they can operate with the very high efficiency needed to account for the very large starburst rate observed.