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 Mg absorbers with the Keck Cosmic Web Imager (KCWI), we discovered a compact group giving rise to a 𝑧 = 2.431 DLA with ultra-strong Mg 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 Lyman series and metal lines such as Mg , Si , and C , 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 (H )/cm −2 ) = 20.53 and an 𝑁 (H )-weighted mean metallicity of log(𝑍/𝑍 ) = −0.68, with a very large Mg linewidth of Δ𝑣 ∼ 700 km s −1 . The highly kinematically complex profile is well-modelled with 30 clouds across low and intermediate ionisation phases with values 13 log(𝑁 (H )/cm −2 ) 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 characterising evolutionary pathways.