Galaxies caught in transition: the role of group environment in shaping the mass-size relation in the local Universe

TL;DR

This study investigates how different galaxy types' sizes relate to their mass in various environments, revealing that transition galaxies are most affected by their surroundings, indicating ongoing structural evolution.

Contribution

It provides the first detailed analysis of the mass-size relation for different galaxy types across environments, highlighting the environmental sensitivity of transition galaxies.

Findings

Early-type and other galaxies show no environmental dependence.

Late-type galaxies are slightly smaller in compact groups.

Transition galaxies exhibit significant size and structural changes in denser environments.

Abstract

The stellar mass-size relation is a sensitive probe of how environment shapes galaxy structure. We analyse this relation in the local Universe for galaxies in compact groups (CGs), low-mass groups ($M_{\rm vir} \leq 10^{13}~M_{\odot}$), and high-mass groups, comparing them to field galaxies using data from the Southern Photometric Local Universe Survey. Galaxies are classified as early types (ETGs; $n \geq 2.5$, $(u-r)_0 \geq 2.3$), late types (LTGs; $n < 2.5$, $(u-r)_0 < 2.3$), transition galaxies (TGs; $n < 2.5$, $(u-r)_0 \geq 2.3$), and others (OGs; $n \geq 2.5$, $(u-r)_0 < 2.3$). We find that ETGs and OGs show no significant environmental dependence: their mass-size slopes and intercepts are statistically consistent across CGs, groups, and the field. LTGs also follow similar relations in the field and in most groups, with only a modest tendency for LTGs in CGs to be smaller at fixed stellar mass. By contrast, TGs display a clear environmental signal: in groups the slope steepens to $\alpha \sim 0.4$ (versus $\alpha \sim 0.2$ in the field) and their sizes are smaller than in the field, with non-overlapping 95\% posterior intervals. These trends suggest that TGs in denser environments are more structurally evolved, likely owing to enhanced bulge prominence and fading of the outer disc, consistent with the S\'ersic-index distributions, which show an excess of TGs with $n_r \gtrsim 1.5$ in groups and CGs. Our findings highlight TGs as an environmentally sensitive population, providing insight into the structural transformation of galaxies in group environments.