Galaxy merger histories and the role of merging in driving star formation at z>1

TL;DR

This study uses cosmological simulations to assess how galaxy mergers influence star formation at redshifts 1 to 4, finding mergers are relatively rare and only mildly enhance star formation, thus playing a limited role in early stellar mass growth.

Contribution

It provides the first detailed analysis of merger fractions, colours, and star formation impacts in massive galaxies during peak cosmic star formation epochs using hydrodynamical simulations.

Findings

Major mergers are about 3% of massive galaxies at these epochs.

Most merging systems at z~1.5 are 'red' with less star formation.

Mergers contribute around a quarter of total star formation, mostly from major mergers.

Abstract

We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1<z<4). The fraction of massive galaxies in major mergers (mass ratio R<4:1) is around 3%, a factor of ~2.5 lower than minor mergers (4:1<R <10:1) at these epochs, with no trend with redshift. At z~1, around a third of massive galaxies have undergone a major merger, while all such systems have undergone either a major or minor merger. While almost all major mergers at z>3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at z<2, with most merging systems at z~1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (~20-40%) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10:1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard LCDM paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.