2017ApJ...843...98R

We report the discovery of a massive log(M/M_☉)=10.74_–0.16^+0.18 galaxy at the same redshift as a carbon-monoxide-bearing sub-damped Lyα absorber (sub-DLA) seen in the spectrum of QSO J1439+1117. The galaxy, J1439B, is located 4.''7 from the QSO sightline, a projected distance of 38 physical kpc at z = 2.4189, and exhibits broad optical emission lines (σ_[OIII]=303±12 km s^–1) with ratios characteristic of excitation by an active galactic nucleus (AGN). The galaxy has a factor of ∼9 lower star formation than is typical of star-forming galaxies of the same mass and redshift. The nearby sub-DLA is highly enriched, suggesting its galactic counterpart must be massive if it follows the z∼2 mass-metallicity relationship. Metallic absorption within the circumgalactic medium of the sub-DLA and J1439B is spread over a velocity range Δv 1000 km s^–1, suggesting an energetic origin. We explore the possibility that a different galaxy could be responsible for the rare absorber, and conclude that it is unlikely based on imaging, integral-field spectroscopy, and high-z massive galaxy pair statistics. We argue that the gas seen in absorption against the QSO was likely ejected from the galaxy J1439B and therefore provides a unique observational probe of AGN feedback in the distant universe.