We report the discovery of a massive \log ( M / M _ { \odot } ) = 10.74 ^ { +0.18 } _ { -0.16 } galaxy at the same redshift as a carbon-monoxide-bearing sub-damped Lyman \alpha absorber ( sub-DLA ) seen in the spectrum of QSO J1439+1117 . The galaxy , J1439B , is located 4 \farcs 7 from the QSO sightline , a projected distance of 38 physical kpc at z = 2.4189 , and exhibits broad optical emission lines ( \sigma _ { [ OIII ] } = 303 \pm 12 km s ^ { -1 } ) with ratios characteristic of excitation by an active galactic nucleus ( AGN ) . The galaxy has a factor of \sim 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 \sim 2 mass-metallicity relationship . Metallic absorption within the circumgalactic medium of the sub-DLA and J1439B is spread over a velocity range \Delta v > 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 .