Galaxy evolution in the post-merger regime. IV -- The long-term effect of mergers on galactic stellar mass growth and distribution

TL;DR

This study analyzes the long-term impact of galaxy mergers on stellar mass growth and distribution, revealing significant, spatially extended mass build-up in post-merger galaxies through a large observational sample.

Contribution

It provides the first direct, model-independent measurements of merger-induced stellar mass growth, highlighting the importance of galaxy structure in star formation distribution.

Findings

Post-mergers show 10-20% stellar mass enhancement in central regions.

Mass enhancement extends out to ~7 kpc or 1 R/R_e.

Mass build-up is significant and spatially extended, independent of stellar population models.

Abstract

Galaxy mergers are known to trigger bursts of central star formation, which should therefore lead to stellar mass growth in their inner regions. However, observational measurements of this `burst mass fraction' are scant. Here, we assemble a large sample of ~14,000 post-coalescence galaxies that have recently completed their merger-induced star formation, and compare various measurements of central stellar mass with a matched control sample. Specifically, we quantify (at fixed redshift, star formation rate and total stellar mass) the stellar mass enhancement within a fixed angular aperture (Delta M_{star,fibre}) and in the galactic bulge (Delta M_{star,bulge}), finding burst mass fractions of 10 -- 20%. 61 galaxies in our sample are at z<0.05 and have integral field unit data from the Mapping Galaxies at Apache Point (MaNGA) survey, allowing further kpc-scale assessment of excess stellar mass and radial gradients. When assessed within apertures defined in units of kpc we again find a ~ 15 -- 20% excess of stellar mass in the central regions of the post-mergers compared with matched controls. However, within apertures defined in units of effective radius this stellar mass enhancement increases to 40%, suggesting that the relative structure/size of the galaxy is important for regulating the location of the merger induced star formation. Moreover, we find that these stellar mass enhancements are spatially extended, out to ~7 kpc or around 1 R/R_e, although the small sample size of the MaNGA overlap limits our radial sampling. Our work represents the first direct measurement of merger-induced stellar mass growth that is independent of stellar population modelling, or fitting light profiles, demonstrating significant and extended mass build-up in late stage post-mergers.