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

TL;DR

This study tests how local group environments modify the stellar mass–size relation by examining galaxies in compact groups, low-/high-mass groups, and the field using S-PLUS data and a hybrid colour–structure classification. The relation is modeled with a Bayesian linear form $\log R_{\mathrm{e}} = \alpha + \beta(\log M_\star - 10.5)$, enabling robust comparisons of slope and intercept across environments for each morphological class. The key finding is that transition galaxies (TGs) show a strong environmental dependence: their mass–size slope is steeper in groups and compact groups ($\beta \approx 0.4$) than in the field ($\beta \approx 0.2$), and they are smaller at fixed mass below $\log M_\star/M_\odot = 10.5$, while the difference fades at higher masses; Sérsic indices $n_r$ also rise in denser environments, indicating higher central concentration. In contrast, ETGs and OGs show little environmental influence, and LTGs are largely similar across environments with only modest size differences in CGs. These results support a scenario where TGs trace environmentally driven structural transformation, likely via bulge growth and outer-disc fading from tidal interactions, minor mergers, and ram-pressure effects, motivating future bulge–disc decompositions and kinematic studies to clarify the underlying processes.

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 $α\sim 0.4$ (versus $α\sim 0.2$ 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 Sérsic-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.

Figures (7)

• Figure 1: Distribution of absolute $r$-band magnitude ($M_r$) as a function of redshift ($z$) for the parent galaxy sample. The gray rectangle marks the redshift interval adopted in this work, $0.035 \le z < 0.096$. The inset shows a magnified view of this region, which defines the final sample analyzed. Points are color-coded by environment: group galaxies (purple), field galaxies (blue), and compact-group (CG) galaxies (orange).
• Figure 2: Density distributions of rest–frame $(g-i)_0$ colour (top-left panel), absolute $i$-band magnitude $M_i$ (top-right panel), and stellar masses (bottom panel) for field galaxies (blue), CGs members (orange), and galaxies in groups (green).
• Figure 3: Classification of ETGs, transition galaxies, and LTGs based on rest–frame colour $(u-r)_0$ and the Sérsic index in the r-band ($n_r$). The vertical guide marks ($n_r=2.5$), and the horizontal guide marks $(u-r)_0=2.2$. A greyscale background displays the two–dimensional kernel–density estimate of the joint distribution, and the marginal normalised histograms are shown along the top $(u-r)_0$ axis and the right $log(n_r)$ axis.
• Figure 4: Relation between effective radius and stellar mass for galaxies in different environments: field (top-left panel), low-mass groups (top-right panel), compact groups (bottom-left panel), and high-mass groups (bottom-right panel). The colour and symbol of the points indicate the morphological type: orange circles for ETGs, blue squares for LTGs, grey triangles for TGs, and purple diamonds for OGs. For each type, the median mass–size relation is shown as a line (solid for ETGs, dashed for LTGs, dash–dotted for TGs, and dotted for OGs), with shaded regions indicating the bootstrap-estimated uncertainties.
• Figure 5: Top panel: Best-fit linear relation between effective radius and stellar mass for TGs in different environments. Bottom-left panel: density distribution of Sérsic index in r-band of TGs for each environment. Bottom-left panel: density distribution of the logarithm of the stellar mass of TGs.