Unveiling the complex circumgalactic medium: a comparative study of merging and non-interacting galaxy groups

Abstract

While most galaxies live in group environments where they undergo an accelerated evolution, the characteristics of their circumgalactic medium (CGM) remain uncertain. We present an analysis of the CGM of two galaxy groups in different stages of interaction: (G1) a close pair of galaxies (z = 0.043) separated by 87 kpc that do not show signs of interactions and (G2) four merging galaxies (z = 0.098) separated by 10 kpc. We present spatially-resolved Keck/KCWI galaxy observations and HST/COS quasar spectra (G1 at 48 kpc and G2 at 100 kpc away) to quantify both the resolved galaxy and CGM properties in these two different group environments. G1 contains two typical star-forming galaxies with no evidence of strong outflows. G2 contains two star-forming, one post-starburst and one quiescent galaxy. Both groups have a range of CGM detected metal lines (H i, C ii, Si ii, Si iii, N v and O vi). Despite G2 being twice as far from the quasar, G2 has $\log (N({\rm H\, {\small \rm I} })/{\rm cm}^{-2})=17.33$, compared to $\log (N(\rm H\, {\small \rm I})/{\rm cm}^{-2})=16.43$ for G1. We find that the CGM of the merging galaxies (G2) is more kinematically complex, is in a higher ionisation state, spans a wider range of metallicities and column densities, has smaller cloud sizes, and is inconsistent with the simple superposition model that seems to match well with G1. We conclude that the complexity of the CGM in merging galaxies surpasses that of not strongly interacting galaxies, suggesting that mergers play a significant role in shaping the intricate structure of the CGM.