2021ApJ...913...18B

We report the serendipitous detection of an H₂-bearing damped Lyα absorber at z = 0.576 in the spectrum of the QSO J0111-0316 in the Cosmic Ultraviolet Baryon Survey. Spectroscopic observations from Hubble Space Telescope-COS in the far-ultraviolet reveal a damped absorber with log[N(H I)/cm^–2] = 20.1 ± 0.2 and log[N(H₂)/cm^–2] =18.97_–0.06^+0.05. The diffuse molecular gas is found in two velocity components separated by Δ ν ≃ 60 km s^–1, with >99.9% of the total H₂ column density concentrated in one component. At a metallicity of ≃50% of solar, there is evidence for Fe enhancement and dust depletion, with a dust-to-gas ratio κ_O ≃ 0.4. A galaxy redshift survey conducted with IMACS and LDSS-3C on Magellan reveals an overdensity of nine galaxies at projected distance d <= 600 proper kpc (pkpc) and line-of-sight velocity offset Δ ν_g <= 300 km s^–1 from the absorber. The closest is a massive, early-type galaxy at d = 41 pkpc that contains ≃70% of the total stellar mass identified at d <= 310 pkpc of the H₂ absorber. The close proximity of the H₂-bearing gas to the quiescent galaxy and the Fe-enhanced chemical abundance pattern of the absorber suggest a physical connection, in contrast to a picture in which DLAs are primarily associated with gas-rich dwarfs. This case study illustrates that deep galaxy redshift surveys are needed to gain insight into the diverse environments that host dense and potentially star-forming gas.