The impact of the dynamical state of galaxy groups on the stellar populations of central galaxies

TL;DR

This study investigates how the dynamical state of galaxy groups influences the stellar populations of their central galaxies, revealing that unrelaxed groups host BGGs with bluer colors, higher star formation rates, and higher metallicities.

Contribution

It introduces a dual-indicator method to classify group relaxedness and links this to observable differences in BGG stellar populations, advancing understanding of galaxy group evolution.

Findings

Unrelaxed groups have BGGs with bluer NUV-r colors.

Unrelaxed groups' BGGs show higher star formation rates.

Higher stellar metallicities are observed in BGGs of unrelaxed groups.

Abstract

We study the stellar populations of the brightest group galaxies (BGGs) in groups with different dynamical states, using GAMA survey data. We use two independent, luminosity dependent indicators to probe the relaxedness of their groups; the magnitude gap between the two most luminous galaxies ($\Delta M_{12}$), and offset between BGG and the luminosity center ($D_{offset}$) of the group. Combined, these two indicators were previously found useful for identifying relaxed and unrelaxed groups. We find that the BGGs of unrelaxed groups have significantly bluer NUV-r colours than in relaxed groups. This is also true at the fixed sersic index. We find the bluer colours cannot be explained away by differing dust fraction, suggesting there are real differences in their stellar populations. SFRs derived from SED-fitting tend to be higher in unrelaxed systems. This is in part because of a greater fraction of BGGs with non-elliptical morphology, but also because unrelaxed systems have larger numbers of mergers, some of which may bring fuel for star formation. The SED-fitted stellar metallicities of BGGs in unrelaxed systems also tend to be higher by around 0.05 dex, perhaps because their building blocks were more massive. We find that the $\Delta M_{12}$ parameter is the most important parameter behind the observed differences in the relaxed/unrelaxed groups, in contrast with the previous study of Trevisan et al. (2017). We also find that groups selected to be unrelaxed using our criteria tend to have higher velocity offsets between the BGG and their group.