A Complex Multiphase DLA Associated with a Compact Group at z=2.431 Traces Accretion, Outflows, and Tidal Streams

Abstract

As part of our program to identify host galaxies of known z=2-3 MgII absorbers with the Keck Cosmic Web Imager (KCWI), we discovered a compact group giving rise to a z=2.431 DLA with ultra-strong MgII absorption in quasar field J234628+124859. The group consists of four star-forming galaxies within 8-28 kpc and $v\sim 40-340$ 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 ($1.66L_{\ast }$, ${\mathrm{SFR}}_{\mathrm{FUV}}=11.6$ M${}_{\odot }$ yr${}^{-1}$) while G2, G3, and G4 are less massive ($0.1-0.3L_{\ast }$, ${\mathrm{SFR}}_{\mathrm{FUV}}=1.4-2.0$ M${}_{\odot }$ yr${}^{-1}$). Using a VLT/UVES quasar spectrum covering the HI Lyman series and metal lines such as MgII, SiIII, and CIV, we characterised the kinematic structure and physical conditions along the line-of-sight with cloud-by-cloud multiphase Bayesian modelling. The absorption system has a total $\log (N(HI)/{\mathrm{cm}}^{-2})=20.53$ and an $N(HI)$-weighted mean metallicity of $\log (Z/Z_{\odot })=-0.68$, with a very large MgII linewidth of $\hat{I}”v\sim 700$ km s${}^{-1}$. The highly kinematically complex profile is well-modelled with 30 clouds across low and intermediate ionisation phases with values $13\lesssim \log (N(HI)/{\mathrm{cm}}^{-2})\lesssim 20$ and $-3\lesssim \log (Z/Z_{\odot })\lesssim 1$. 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 characterising evolutionary pathways.

29 pages, 10 figures. Accepted for publication in MNRAS 28 June 2022