The limited role of galaxy mergers in driving stellar mass growth over cosmic time

TL;DR

This study uses cosmological simulations to show that galaxy mergers contribute only a small fraction to stellar mass growth over cosmic time, with smooth accretion being the dominant process.

Contribution

It provides a comprehensive quantification of merger contributions to stellar mass growth using Horizon-AGN simulation, challenging the significance of mergers in galaxy evolution.

Findings

Mergers enhance star formation similarly regardless of size.

Only about 25% of stellar mass is due to mergers over cosmic time.

Smooth accretion is the main driver of stellar mass growth.

Abstract

A key unresolved question is the role that galaxy mergers play in driving stellar mass growth over cosmic time. Recent observational work hints at the possibility that the overall contribution of `major' mergers (mass ratios $\gtrsim$1:4) to cosmic stellar mass growth may be small, because they enhance star formation rates by relatively small amounts at high redshift, when much of today's stellar mass was assembled. However, the heterogeneity and relatively small size of today's datasets, coupled with the difficulty in identifying genuine mergers, makes it challenging to $\textit{empirically}$ quantify the merger contribution to stellar mass growth. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation, to comprehensively quantify the contribution of mergers to the star formation budget over the lifetime of the Universe. We show that: (1) both major and minor mergers enhance star formation to similar amounts, (2) the fraction of star formation directly attributable to merging is small at all redshifts (e.g. $\sim$35 and $\sim$20 per cent at z$\sim$3 and z$\sim$1 respectively) and (3) only $\sim$25 per cent of today's stellar mass is directly attributable to galaxy mergers over cosmic time. Our results suggest that smooth accretion, not merging, is the dominant driver of stellar mass growth over the lifetime of the Universe.