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 tackles the problem of quantifying how much stellar mass growth in galaxies is driven by merger-induced star formation, distinct from secular growth. It assembles a large post-merger sample (~14,000 quenched, late-stage systems) identified by the mummi classifier and compares them to carefully matched controls across fibre, bulge, and MaNGA-resolved apertures to measure the burst mass fraction. The results show burst mass fractions of roughly 10–20% within fixed angular apertures and bulges, with a similar ∼15–20% excess extending out to about 7 kpc in MaNGA maps, indicating extended stellar mass growth beyond the very center. This direct, population-wide measurement does not rely on stellar population modelling or light-profile fitting and aligns with recent SFR and molecular-gas findings, implying merger-driven star formation contributes substantial mass growth over kiloparsec scales and should be incorporated into galaxy evolution 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 (~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.

Figures (5)

• Figure 1: Normalized distribution of fibre stellar mass offset ($\Delta M_{\star,fibre}$) for late-stage (0.96 $< T_{PM} < 1.76$ Gyr,) quenched post-mergers (red histogram) and non-mergers (blue). The vertical dashed line is at zero. The median $\Delta M_{\star,fibre}$ for the post-mergers is 0.04 dex, corresponding to a $\sim$ 10 % enhancement.
• Figure 2: The median values $\Delta M_{\star,fibre}$ for the post-mergers computed in bins of redshift and stellar mass. The enhancement in fibre stellar mass for post-mergers seen in Figure \ref{['dmfibre']} is present across the full stellar mass and redshift range of the sample.
• Figure 3: Normalized distribution of bulge stellar mass offset ($\Delta M_{\star,bulge}$) for late-stage (0.96 $< T_{PM} < 1.76$ Gyr,) quenched post-mergers (red histogram) and non-mergers (blue). The vertical dashed line is at zero. The median $\Delta M_{\star,bulge}$ for the post-mergers is 0.09 dex, corresponding to a $\sim$ 20 % enhancement.
• Figure 4: The excess mass measured using the 61 $z\le$0.05 quenched late-stage mummi post-mergers with MaNGA observations. The median mass excess is computed in apertures of increasing size relative to the five best matched control galaxies. Error bars are calculated as the standard error on the median. We find that, for apertures with sizes less than $\sim$ 7 kpc, there is an approximately 15 -- 20 % mass enhancement in the post-mergers.
• Figure 5: The median stellar mass excess measured in annuli. Error bars are calculated as the standard error on the median. Significant stellar mass excesses are measured in annuli up to 7 kpc from the galaxy centre.