The Mrk 231 molecular outflow as seen in OH^⋆

¹ Universidad de AlcaláDepartamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
e-mail: eduardo.gonzalez@uah.es
² Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington DC 20375, USA
³ Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstraße 1, 85748 Garching, Germany
⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Cornell University, Astronomy Department, Ithaca NY 14853, USA
⁷ University of Oxford, Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

Received: 16 April 2013
Accepted: 2 October 2013

Abstract

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 ²Π_3/2J= 9/2 − 7/2 OH doublet at 65 μm, 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 ~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, ≳70 × (2.5 × 10^-6/X_OH) M_⊙ yr^-1 sr^-1, where X_OH is the OH abundance relative to H nuclei. In spherical symmetry, this would correspond to ≳850 × (2.5 × 10^-6/X_OH) M_⊙ yr^-1, though significant collimation is inferred from the line profiles. The momentum flux of the excited component attains ~15 L_AGN/c, with an OH column density of (1.5−3) × 10¹⁷ cm^-2 and a mechanical luminosity of ~10¹¹L_⊙. 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 ¹⁸OH enhancement, with OH/¹⁸OH ≲ 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.