Galaxy evolution in compact groups - III. Structural analysis of galaxies and dynamical state of non-isolated compact groups

TL;DR

This study investigates how non-isolated compact groups (non-isolated CGs) embedded in larger structures influence galaxy evolution, separating local CG effects from large-scale environment. Using MorphoPLUS to fit multi-band Sérsic profiles across 12-band S-PLUS data, the authors quantify structural parameters ($n$, $R_e$) for 392 CG galaxies and 973 surrounding-group galaxies within 102 major structures, complemented by SFR and stellar mass estimates from the GSWLC. They find higher quenched and ETG fractions in non-isolated CGs, especially at $\log(M/M_\odot) > 10.2$, and identify a distinct $R_e$–$n$ signature for transition galaxies in CGs, consistent with accelerated morphological evolution. Phase-space analyses reveal many projected CGs in clusters are not physically bound, highlighting projection effects and a spectrum of infall stages; overall, the compact CG configuration drives evolutionary paths beyond the large-scale environment.

Abstract

Compact Groups (CGs) of galaxies are dense systems where projected separations are comparable to their optical diameters. A subset - non-isolated CGs - are embedded within major structures. Using multi-band S-PLUS data, we analyse galaxies in 122 non-isolated CGs within more massive systems such as larger groups and clusters. We compare them to galaxies in the host structures, hereafter surrounding group galaxies. Structural parameters were obtained with MorphoPLUS, a pipeline for multi-wavelength Sérsic profile fitting. Dividing galaxies into early (ETG), transition, or late types (LTG), we find: (1) Non-isolated CGs host higher quenched fractions and more ETGs, especially for stellar masses $\log(M/M_\odot) > 10.2$, than surrounding groups. (2) Sérsic indices increase with wavelength for all morphological types in both environments, whereas effective radii show a stronger morphology-dependent behaviour - ETGs become more compact towards redder bands, while LTGs exhibit flatter $Re(λ)$ trends. Environmental differences remain weak, with only a modest enhancement of the gradients for ETGs in non-isolated CGs. (3) Transition galaxies in CGs show a concentrated $R_e$-$n$ distribution and faint-end bimodality, consistent with ongoing morphological transformation absent in surrounding groups. (4) Phase-space analysis indicates that some CGs in clusters are projection artefacts, while others are genuine dense systems at various infall stages, from recent arrivals to ancient remnants. These results show that galaxies in non-isolated CGs follow distinct evolutionary paths compared to their surrounding groups galaxies, suggesting that the compact configuration plays a unique role beyond the influence of the larger-scale environment.

Figures (16)

• Figure 1: Left: normalised histogram distribution of $M_r$ shows the galaxies in non-isolated CGs represented in red, while those in the surrounding groups (in which the CGs are embedded) are shown in purple. In the latter case, we excluded the galaxies that are part of the CGs. The absolute magnitudes in the $r$-band have been corrected for galactic extinction and include the K-correction. Right: normalised redshift ($z$) distributions for the same samples.
• Figure 2: 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 $(u-r)_0=2.2$. Purple points show surrounding–group galaxies and red points show galaxies in non–isolated CGs. 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 3: Distributions of projected local density $(\Sigma_5)$. The distributions for the isolated CGs, Non-isolated CGs, and surrounding group galaxies samples are shown in orange-, red- and purple-colour lines, respectively. Vertical lines show the medians of the samples.
• Figure 4: Host-structure properties. Left: velocity dispersion estimated in this work. Right: halo mass from 2007Yang. The broad ranges arise because some hosts are small substructures with few members, whereas others contain $>50$ galaxies, leading to higher $\sigma_G$ and larger $M_{halo}$.
• Figure 5: Top panels: Median Sérsic index ($n$) as a function of wavelength for ETGs (red/orange), LTGs (blue/cyan), and TGs (green/light green) in non-isolated compact groups (circles) and their surrounding groups (squares). Bottom panels: Median effective radius ($R_e$) as a function of wavelength for the same subsamples. The three panels correspond to decreasing luminosity bins from left to right. Error bars show the 90% confidence intervals estimated by bootstrapping.