Star-forming compact groups: Tracing the early evolutionary stages of compact group environments

Abstract

In the context of pre-processing -- a scenario in which galaxies quench their star formation within substructures before falling into clusters -- we investigate the impact of environment on the physical and morphological properties of galaxies in Compact Groups (CGs), focusing specifically on a sample of Star-Forming Compact Groups (SFCGs). Our aim is to characterize the physical and morphological properties of galaxies in SFCGs, analogues of the Blue Infalling Group, and to understand how the environment influences their evolution. We use photometric techniques to derive stellar masses and star formation rates (SFRs). Morphological parameters are extracted from DECaLS images, obtaining parametric properties such as the S\'ersic index ($n$) and effective radius ($R_{\mathrm{e}}$) using GALFITM, as well as non-parametric indices -- including the Gini coefficient, $M_{20}$, and asymmetry -- from the same data. These indicators allow us to classify galaxies into E/S0/Sa, Sb/Sc/Ir, and merger types. All measurements are compared to a control sample of field galaxies to assess environmental effects. We find no significant differences in $n$ and $R_{\mathrm{e}}$ between SFCG and field galaxies, in contrast to results reported for other CG samples. However, SFCG galaxies exhibit higher specific star formation rates (sSFRs) than their field counterparts. Approximately $16\%$ of SFCG galaxies show merger features and elevated asymmetry. These mergers also present enhanced SFRs compared to both other SFCG types and the field population. We propose that SFCGs represent an earlier evolutionary phase of CGs, supported by their lower velocity dispersions and moderate crossing times, in addition to the observed SFR enhancement and the absence of pronounced morphological transformation. Galaxy mergers in this phase appear to enhance, rather than suppress, star formation.