DIAGNOSTICS OF AGN-DRIVEN MOLECULAR OUTFLOWS IN ULIRGs FROM<i>HERSCHEL</i>-PACS OBSERVATIONS OF OH AT 119 μm

Abstract

We report on our observations of the 79 and 119um doublet transitions of OH\nfor 24 local (z&lt;0.262) ULIRGs observed with Herschel-PACS as part of the\nHerschel ULIRG Survey (HERUS). Some OH119 profiles display a clear P-Cygni\nshape and therefore imply outflowing OH gas, other profiles are predominantly\nin absorption or are completely in emission. We find that the relative strength\nof the OH emission component decreases as the silicate absorption increases.\nThis locates the OH outflows inside the obscured nuclei. The maximum outflow\nvelocities for our sources range from less than 100 to 2000 km/s, with 15/24\n(10/24) sources showing OH absorption at velocities exceeding 700 km/s (1000\nkm/s). Three sources show maximum OH outflow velocities exceeding that of\nMrk231. Since outflow velocities above 500-700 km/s are thought to require an\nactive galactic nucleus (AGN) to drive them, about 2/3 of our ULIRG sample may\nhost AGN-driven molecular outflows. This finding is supported by the\ncorrelation we find between the maximum OH outflow velocity and the IR-derived\nbolometric AGN luminosity. No such correlation is found with the IR-derived\nstar formation rate. The highest outflow velocities are found among sources\nwhich are still deeply embedded. We speculate that the molecular outflows in\nthese sources may be in an early phase of disrupting the nuclear dust veil\nbefore these sources evolve into less obscured AGN. Four of our sources show\nhigh-velocity wings in their [C II] fine-structure line profiles implying\nneutral gas outflow masses of at least 2-4.5 x 10^8 Msun.\n