Mergers of galaxies

TL;DR

Galaxy mergers play a crucial role in the evolution of massive galaxies, transforming their structure, fueling star formation and black hole growth, with the merger fraction increasing up to redshift 6.

Contribution

This paper reviews observational methods for detecting galaxy mergers and discusses their impact on galaxy morphology and growth over cosmic time.

Findings

Merger fraction increases with redshift, up to z~6.

Mergers transform galaxy morphology from rotational to dispersion support.

Only about one-third of stellar and black hole mass is due to mergers.

Abstract

The evolution of our Universe is strongly influenced by the attractive force of gravity. A key aspect of this evolution, therefore, is the merging of galaxies. Here, we explore the role of mergers in shaping the properties of massive galaxies over cosmic time. Observational methods of finding mergers include identifying galaxy pairs in close proximity, visual inspection of galaxy images to identify signatures of mergers (e.g. tidal features) and using morphological parameters such as Asymmetry and the Gini coefficient. The fraction of merging galaxies increases with redshift, potentially out to z~6. The principal impact of merging is to transform the morphological mix of massive galaxies, from largely rotationally-supported systems at high redshift to more dispersion-dominated systems in the nearby Universe. Mergers also drive gas towards the central regions of the remnant, fuelling starbursts and feeding supermassive black holes. However, only around a third of the stellar and black hole mass at the present day is directly attributable to merging.