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

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 (~14,000) sample of 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. Again, we find a ~15 -- 20 % excess of stellar mass in the central regions of the post-mergers compared with matched controls. However, contrary to previous works, which have inferred very compact, centralized merger-induced mass growth, we find a 15 % stellar mass excess out to ~7 kpc (1.4 R_e for the stellar masses in our sample). Our work represents the first direct measurement of merger-induced stellar mass that is independent of stellar population modelling, or fitting light profiles, demonstrating significant and extended mass build-up in late stage post-mergers.