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 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*, SFRFUV = 11.6 M⊙ yr−1) while G2, G3, and G4 are less massive (0.1–0.3L*, SFRFUV = 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.

Host galaxies of ultrastrong Mg ii absorbers at z ∼ 0.5

“…Gauthier (2013) have identified a z ∼ 0.5 galaxy group dominated by passive galaxies associated with a USMgii absorption system and argued that such strong absorption are driven by cool intragroup gas rather than star-formation-driven outflows. Nielsen et al (2022) have identified a compact galaxy group associated with a DLA, which is also a USMgii system at z ∼ 2.4, using KCWI and argued the gas in absorption is related to star-formation driven outflows, accretion from the IGM, and the tidal streams due to the galaxy-galaxy interactions. Some of the deviations shown by USMgii absorbers with respect to the relationships found for the general population of Mg ii absorbers could be explained if a good fraction of USMgii absorbers is not originating from the CGM of individual galaxies.…”

“…For a fully saturated Mg ii line a 3Å limit on W2796 corresponds to a minimum velocity width of 320 kms −1 . Such large velocity spreads could be caused by (i) galactic-scale outflows (Nestor et al 2011a), (ii) filamentary accretion onto galaxies (Rubin et al 2012), and (iii) galaxy mergers (Rubin et al 2010) and intra-group gas (Gauthier 2013;Nielsen et al 2022;Zou et al 2018). In such instances, one could use the measured metallicities (Lehner et al 2013) and galaxy orientations relative to quasar sightlines to differentiate between the various possibilities (Kacprzak et al 2012).…”