Galaxy evolution in compact groups

Compact groups (CGs) of galaxies are an extreme environment for the morphological transformations and the cessation of star formation in galaxies. However, despite initially being conceived as isolated systems, it is now widely recognised that many of them are not as isolated as expected. Our objective is to understand the dynamics of CGs, as well as how the environment surrounding CGs impacts their morphological and physical properties. To achieve this, we selected a sample of 316 CGs in the Stripe 82 region, with a total of 1011 galaxies, and a sample of 2281 field galaxies as a control sample. We find that at least 41% of our sample of CGs are part of major structures, i.e. non-isolated CGs. We find a bimodality in the effective radius ( R e )-Sérsic index ( n ) plane for all transition galaxies (those with ( u − r ) > 2.3 and n < 2.5) in CGs. Additionally, transition galaxies in isolated CGs populate more densely the R e − n plane for n < 1.75. In contrast, transition galaxies in non-isolated CGs show a bimodal distribution in the R e − n plane, with the n values smoothly increasing towards higher values, and 62% of these galaxies having n > 1.5. This indicates that the majority of these galaxies have already undergone a morphological transformation and primarily contribute to the population of more compact galaxies in the R e − n plane. We find that galaxies in our sample of CGs have a lower mean specific star formation rate (sSFR) compared to the control sample, with non-isolated CGs showing even lower sSFR values, indicating that dense environments suppress star formation. Additionally, non-isolated CGs have a higher fraction of quenched galaxies relative to isolated CGs and the control sample. Based on our results, we propose an evolutionary scenario where the major structures in which the CGs are embedded accelerate the morphological transformations of their galaxy members, and also facilitates preprocessing. Our findings highlight the importance of considering the larger structures in which CGs may be located, when analysing the properties of their galaxy members, as this can significantly affect the evolution of CGs and their galaxies.