The Mrk 231 molecular outflow as seen in OH

We report on the Herschel/PACS observations of OH in Mrk 231, with detections in nine doublets observed within the PACS range, and present radiative-transfer models for the outflowing OH. Clear signatures of outflowing gas are found in up to six OH doublets with different excitation requirements. At least two outflowing components are identified, one with OH radiatively excited, and the other with low excitation, presumably spatially extended and roughly spherical. Particularly prominent, the blue wing of the absorption detected in the in-ladder (2)Pi(3/2) J = 9/2-7/2 OH doublet at 65 mu m, with E-lower = 290 K, indicates that the excited outflowing gas is generated in a compact and warm (circum) nuclear region. Because the excited, outflowing OH gas in Mrk 231 is associated with the warm, far-infrared continuum source, it is most likely more compact (diameter of similar to 200-300 pc) than that probed by CO and HCN. Nevertheless, its mass-outflow rate per unit of solid angle as inferred from OH is similar to that previously derived from CO, greater than or similar to 70 x (2.5 x 10(-6)/X-OH) M-circle dot yr(-1) sr(-1), where X-OH is the OH abundance relative to H nuclei. In spherical symmetry, this would correspond to greater than or similar to 850 x (2.5 x 10(-6)/X-OH) M-circle dot yr(-1), though significant collimation is inferred from the line profiles. The momentum flux of the excited component attains similar to 15 L-AGN/c, with an OH column density of (1.5-3) x 10(17) cm(-2) and a mechanical luminosity of similar to 10(11) L-circle dot. In addition, the detection of very excited, radiatively pumped OH peaking at central velocities indicates the presence of a nuclear reservoir of gas rich in OH, plausibly the 130 pc scale circumnuclear torus previously detected in OH megamaser emission, that may be feeding the outflow. An exceptional (OH)-O-18 enhancement, with OH/(OH)-O-18 less than or similar to 30 at both central and blueshifted velocities, is most likely the result of interstellar-medium processing by recent starburst and supernova activity within the circumnuclear torus or thick disk.