Galaxy evolution in compact groups II. Witnessing the influence of major structures in their evolution

Abstract

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 340 CGs in the Stripe 82 region, with a total of 1083 galaxies, and a sample of 2281 field galaxies as a control sample. We find that at least 27% of our sample of CGs are part of major structures, i.e. non-isolated CGs. We find a bimodality in the effective radius (Re)-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 Re − n plane for n < 1.75. In contrast, transition galaxies in non-isolated CGs have smoothly increasing n values, suggesting these galaxies have already suffered morphological transformation, and primarily contribute to the distribution of more compact galaxies in the Re−n plane for all transition galaxies in CGs. We also find significant differences in the specific star-formation rate (sSFR) distribution between the late-type galaxies (LTGs) (those with (u − r) < 2.3 and n < 2.5) in non-isolated CGs and the same type of galaxies in the control sample, suggesting that the evolution of LTGs differs in non-isolated CGs. Moreover, Early-type galaxies (those with (u − r) > 2.3 and n > 2.5) and transition galaxies in non-isolated CGs have lower sSFR values and a higher fraction of quenched galaxies, compared to those in isolated CGs. 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.