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

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.

💡 Research Summary

This paper investigates how the local environment influences the stellar mass‑size relation of galaxies, focusing on a population that may be undergoing structural transformation. Using data from the Southern Photometric Local Universe Survey (S‑PLUS) DR4, the authors construct three environmentally distinct samples in the redshift range 0.035 < z < 0.096: compact groups (CGs), low‑mass and high‑mass galaxy groups (derived from the 2PIGG catalogue), and a control sample of isolated field galaxies. After matching absolute‑magnitude and redshift distributions, the final dataset comprises 595 CG members, 2 402 group galaxies, and 915 field galaxies, all with reliable photometry in twelve optical bands.

Structural parameters (effective radius Re and Sérsic index n) are measured in the r‑band using the MorphoPLUS pipeline, which fits multi‑band Sérsic models with GALFITM. Stellar masses are estimated via the colour‑based prescription of Taylor et al. (2011), calibrated against i‑band absolute magnitudes and (g‑i)₀ colours; the authors verify that these masses are consistent with SED‑derived masses to within ~0.13 dex scatter and ~0.2 dex systematic uncertainty.

Galaxies are classified on the (u‑r)₀–log n plane using data‑driven thresholds (u‑r)₀ = 2.2 and n = 2.5, yielding four categories: 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). The morphological mix varies with environment: ETGs dominate in groups and CGs (≈40–50 % of each sample), while LTGs are the majority in the field (≈50 %). TGs constitute ~17 % of the field but only ~10–12 % of groups and CGs.

The mass‑size relation is modeled as log Re = α log M* + β. Bayesian linear regression provides posterior distributions for slope (α) and intercept (β), and 95 % credible intervals are used to assess environmental differences. The key findings are:

1. ETGs and OGs: Across all environments, these classes share statistically indistinguishable slopes (α ≈ 0.6) and intercepts (β ≈ −5.0). Their 95 % credible intervals overlap, indicating that dense environments do not measurably alter the size growth of already spheroid‑dominated systems.

2. LTGs: Field and group LTGs exhibit similar slopes (α ≈ 0.3) and intercepts, suggesting that the canonical inside‑out growth of disc galaxies proceeds largely independently of environment. In compact groups, however, LTGs are modestly (~10 %) smaller at fixed stellar mass, hinting at mild disc truncation or tidal stripping.

3. Transition Galaxies (TGs): This class shows the strongest environmental signal. Field TGs have a shallow slope (α ≈ 0.2) and larger intercept (β ≈ −4.2). In both low‑mass and high‑mass groups, the slope steepens to α ≈ 0.4 and the intercept drops to β ≈ −4.8, meaning that at a given stellar mass TGs in groups are 20–30 % smaller than their field counterparts. The 95 % credible intervals for group TGs and field TGs do not overlap, confirming a statistically significant difference.

The authors also examine the Sérsic‑index distribution of TGs. In groups and CGs there is an excess of TGs with n ≳ 1.5 compared to the field, indicating that these galaxies have developed more prominent central concentrations while their outer discs are fading or being stripped. This pattern is interpreted as evidence of “bulge growth + disc fading” driven by processes typical of group environments: repeated weak mergers, tidal interactions, and gas removal (e.g., ram‑pressure stripping or starvation). Such mechanisms can accelerate the morphological transformation from star‑forming discs to quiescent spheroids, positioning TGs as a transitional population that is especially sensitive to the pre‑processing that occurs in groups before galaxies fall into massive clusters.

The paper situates its results within the broader literature. Earlier works (e.g., Poggiani et al. 2013; Cebrián & Trujillo 2014) reported smaller galaxy sizes in dense regions, while others (Rettura et al. 2010; Huertas‑Compán et al. 2013) found little environmental dependence. The present study reconciles these discrepancies by showing that when the full galaxy population is considered, environmental effects can be diluted; however, when focusing on the transitional subset, the signal becomes pronounced.

In conclusion, the stellar mass‑size relation is a powerful diagnostic of environmental processing, but its sensitivity depends on galaxy type. Transition galaxies exhibit clear size suppression and steeper scaling in groups, highlighting the role of group‑scale interactions in driving structural evolution. The authors suggest that future work should combine high‑resolution imaging and integral‑field spectroscopy to dissect the stellar‑population gradients and kinematics of TGs, thereby directly probing the physical mechanisms (bulge growth, disc truncation, quenching) responsible for the observed trends. This will refine models of galaxy evolution that currently treat groups as a simple intermediate step between the field and clusters, emphasizing instead their active role in reshaping galaxy structure.