An enhanced merger fraction within the galaxy population of the SSA22 protocluster at z ~ 3.1

TL;DR

This study finds a higher galaxy merger rate in the SSA22 protocluster at z~3.1 compared to the field, suggesting mergers play a key role in accelerated galaxy evolution in dense environments.

Contribution

It provides the first morphological classification-based evidence of an enhanced merger fraction in a high-redshift protocluster environment.

Findings

Merger fraction in SSA22 is 48% compared to 30% in the field.

Enhanced merger rate suggests mergers drive accelerated growth in protoclusters.

Results imply mergers contribute to star formation and black hole activity in dense regions.

Abstract

The over-dense environments of protoclusters of galaxies in the early Universe (z>2) are expected to accelerate the evolution of galaxies, with an increased rate of stellar mass assembly and black hole accretion compared to co-eval galaxies in the average density `field'. These galaxies are destined to form the passive population of massive systems that dominate the cores of rich clusters today. While signatures of accelerated growth of galaxies in the SSA22 protocluster (z=3.1) have been observed, the mechanism driving this remains unclear. Here we show an enhanced rate of galaxy-galaxy mergers could be responsible. We morphologically classify Lyman-break Galaxies (LBGs) in the SSA22 protocluster and compare these to those of galaxies in the field at z=3.1 as either active mergers or non-merging using Hubble Space Telescope ACS/F814W imaging, probing the rest-frame ultraviolet stellar light. We measure a merger fraction of 48+/-10 per cent for LBGs in the protocluster compared to 30+/-6 per cent for the field. Although the excess is marginal, an enhanced rate of mergers in SSA22 hints that galaxy-galaxy mergers are one of the key channels driving accelerated star formation and AGN growth in protocluster environments.