Diagnostics of AGN-driven Molecular Outflows in ULIRGs from
  Herschel-PACS Observations of OH at 119um
  We report on our observations of the 79 and 119um doublet transitions of OH
for 24 local (z<0.262) ULIRGs observed with Herschel-PACS as part of the
Herschel ULIRG Survey (HERUS). Some OH119 profiles display a clear P-Cygni
shape and therefore imply outflowing OH gas, other profiles are predominantly
in absorption or are completely in emission. We find that the relative strength
of the OH emission component decreases as the silicate absorption increases.
This locates the OH outflows inside the obscured nuclei. The maximum outflow
velocities for our sources range from less than 100 to 2000 km/s, with 15/24
(10/24) sources showing OH absorption at velocities exceeding 700 km/s (1000
km/s). Three sources show maximum OH outflow velocities exceeding that of
Mrk231. Since outflow velocities above 500-700 km/s are thought to require an
active galactic nucleus (AGN) to drive them, about 2/3 of our ULIRG sample may
host AGN-driven molecular outflows. This finding is supported by the
correlation we find between the maximum OH outflow velocity and the IR-derived
bolometric AGN luminosity. No such correlation is found with the IR-derived
star formation rate. The highest outflow velocities are found among sources
which are still deeply embedded. We speculate that the molecular outflows in
these sources may be in an early phase of disrupting the nuclear dust veil
before these sources evolve into less obscured AGN. Four of our sources show
high-velocity wings in their [C II] fine-structure line profiles implying
neutral gas outflow masses of at least 2-4.5 x 10^8 Msun.
