Galaxy And Mass Assembly (GAMA): Galaxy close-pairs, mergers, and the future fate of stellar mass

TL;DR

This study analyzes the dependence of galaxy close-pairs and mergers on stellar mass using GAMA-II data, revealing the fraction of mass accreted through mergers and their impact on galaxy evolution.

Contribution

It provides a detailed characterization of the stellar mass dependence of galaxy mergers and introduces a new fit for the evolution of close-pair fractions with redshift.

Findings

Mass accretion onto larger galaxies is 2.0%-5.6%.

No significant change in close-pair fraction from z=0.05 to 0.2.

Mergers deepen the GSMF dip and increase Mstar by 0.01-0.05 dex.

Abstract

We use a highly complete subset of the GAMA-II redshift sample to fully describe the stellar mass dependence of close-pairs and mergers between 10^8 Msun and 10^12 Msun. Using the analytic form of this fit we investigate the total stellar mass accreting onto more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging onto more massive companions is 2.0%-5.6%. Using the GAMA-II data we see no significant evidence for a change in the close-pair fraction between redshift $z = 0.05-0.2$. However, we find a systematically higher fraction of galaxies in similar mass close-pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function $\gamma_M =A(1+z)m$ to predict the major close-pair fraction, we find fitting parameters of $A = 0.021 \pm 0.001$ and $m = 1.53 \pm 0.08$, which represents a higher low-redshift normalisation and shallower power-law slope than recent literature values. We find that the relative importance of in-situ star-formation versus galaxy merging is inversely correlated, with star-formation dominating the addition of stellar material below Mstar and merger accretion events dominating beyond Mstar. We find mergers have a measurable impact on the whole extent of the GSMF, manifest as a deepening of the dip in the GSMF over the next Gyr and an increase in Mstar by as much as 0.01-0.05 dex.