Galaxies Caught in Transition: The Role of Group Environment in Shaping the Mass–Size Relation in the Local Universe

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 vir ≤ 10 13 M ⊙ ), and high-mass groups, comparing them to field galaxies using data from the Southern Photometric Local Universe Survey. Galaxies are classified as early-type galaxies (ETGs; n ≥ 2.5, ( u − r ) 0 ≥ 2.2), late-type galaxies (LTGs; n < 2.5, ( u − r ) 0 <2.2), transition galaxies (TGs; n < 2.5, ( u − r ) 0 ≥ 2.2), and other galaxies (OGs; n ≥ 2.5, ( u − r ) 0 <2.2). 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 α ∼ 0.4 (versus α ∼ 0.2 in the field) and their sizes are smaller than in the field, with nonoverlapping 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 disk, consistent with the Sérsic-index distributions, which show an excess of TGs with n r ≳ 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.