  We report the detection of an emission feature at the 12 sigma level with
FWHM line width of about 450 km/s toward the nearest quasi-stellar object, QSO
Mrk 231. Based on observations with the IRAM 30 m telescope and the NOEMA
Interferometer, the 11-10 transition of molecular oxygen is the likely origin
of line with rest frequency close to 118.75 GHz. The velocity of the O2
emission in Mrk 231 coincides with the red wing seen in CO emission, suggesting
that it is associated with the outflowing molecular gas, located mainly at
about ten kpc away from the central AGN. This first detection of extragalactic
molecular oxygen provides an ideal tool to study AGN-driven molecular outflows
on dynamic time scales of tens of Myr. O2 may be a significant coolant for
molecular gas in such regions affected by AGN-driven outflows. New
astrochemical models are needed to explain the implied high molecular oxygen
abundance in such regions several kpc away from the center of galaxies.
