Compact groups from semi-analytical models of galaxy formation

Abstract

Compact groups (CGs) of galaxies have proven to be unique environments for studying galaxy interactions. However, there is a need for studies based on statistical evidence to predict how the two main galaxies in these systems went on to develop the properties we observe today. We propose a detailed analysis of the galaxy evolution to disentangle the relationship between the two first-ranked galaxies in CGs throughout their history as a function of the assembly channels of their hosts. Our study was performed from a semi-analytical point of view, using more than 20 000 CGs extracted from mock catalogues built from four different semi analytical models of galaxy formation. We based our analysis on studying the first- (1R) and secondranked (2R) galaxies in CGs, where the ranking is determined using the galaxy stellar mass as the parameter. The 1R galaxies have significantly reduced their star-forming capacity over time, reaching a quenching stage and often becoming bulge-dominated or elliptical. Notably, this transformation occurred earlier for 1R galaxies in early-formed CGs (around 5 to 8 Gyrs ago), while those in recently-formed CGs experienced this change more recently (around 2 to 3 Gyrs ago). The analysis of the time evolution of a variant of the Tremaine & Richstone statistics showed that the 1R galaxy in early-formed CGs began to stand out for its dominant properties around 6 Gyrs ago, almost 5 Gyrs earlier than the 1R inhabiting recently formed CGs. Our merger trees analysis has demonstrated that 1R and 2R galaxies are easily differentiated by their galaxy interactions during their life span. A large majority of the 1R galaxies have experienced at least one major merger event during their life, while we observed this only for a third of 2R galaxies. The 1R galaxies can also display several of these events and most of their last major merger (LMM) events can be described as the addition of a progenitor that is the second most massive galaxy in their surroundings at the time of the merger. We find that the semi-analytical framework explored in this work describes a scenario where galaxy mergers are the main driving force in shaping the properties of the 1R galaxies in CGs. We note that this scenario is especially intensive when those galaxies inhabit CGs that had formed early on.