A complex multiphase DLA associated with a compact group at z = 2.431 traces accretion, outflows, and tidal streams

As part of our program to identify host galaxies of known z = 2-3 Mg II absorbers with the Keck Cosmic Web Imager (KCWI), we discovered a compact group giving rise to a z = 2.431 DLA with ultrastrong Mg II absorption in quasar field J234628+124859. The group consists of four star-forming galaxies within 8-28 kpc and v ~ 40-340 km s^-1 of each other, where tidal streams are weakly visible in deep HST imaging. The group geometric centre is D = 25 kpc from the quasar (D = 20-40 kpc for each galaxy). Galaxy G1 dominates the group (1.66L_*, SFR_FUV = 11.6 M_⊙ yr^-1) while G2, G3, and G4 are less massive (0.1-0.3L_*, SFR_FUV = 1.4-2.0 M_⊙ yr^-1). Using a VLT/UVES quasar spectrum covering the H I Lyman series and metal lines such as Mg II, Si III, and C IV, we characterized the kinematic structure and physical conditions along the line of sight with cloud-by-cloud multiphase Bayesian modelling. The absorption system has a total $\log (N({{{\rm H}\,\rm{\small I}}})/{\rm cm}^{-2})=20.53$ and an $N({{{\rm H}\,\rm{\small I}}})$-weighted mean metallicity of log (Z/Z_⊙) = -0.68, with a very large Mg II linewidth of Δv ~ 700 km s^-1. The highly kinematically complex profile is well modelled with 30 clouds across low- and intermediate-ionization phases with values ${13\lesssim \log (N({{{\rm H}\,\rm{\small I}}})/{\rm cm}^{-2})\lesssim 20}$ and -3 ≲ log (Z/Z_⊙) ≲ 1. Comparing these properties to the galaxy properties, we infer a wide range of gaseous environments, including metal-rich outflows, metal-poor IGM accretion, and tidal streams from galaxy-galaxy interactions. This diversity of structures forms the intragroup medium around a complex compact group environment at the epoch of peak star formation activity. Surveys of low-redshift compact groups would benefit from obtaining a more complete census of this medium for characterizing evolutionary pathways.