Detection of HCN, HCO+, and HNC in the Mrk 231 molecular outflow : Dense molecular gas in the AGN wind

Abstract

Aims. Our goal is to study gas properties in large-scale molecular outflows and winds from active galactic nuclei (AGNs) and starburst galaxies. Methods. We obtained high-resolution (1′′.55 × 1′′.28) observations of HCN, HCO+, HNC 1-0 and HC3N 10-9 of the ultraluminous galaxy (ULIRG) Mrk 231 with the IRAM Plateau de Bure Interferometer. Results. We detect luminous emission from HCN, HCO+ and HNC 1-0 in the QSO ULIRG Mrk 231. All three lines show broad line wings-which are particularly prominent for HCN. Velocities are found to be similar (≈± 750 km s-1) to those found for CO 1-0. This is the first time bright HCN, HCO+ and HNC emission has been detected in a large-scale galactic outflow. We find that both the blue-and red-shifted line wings are spatially extended by at least 0′′.75 (>700 pc) in a north-south direction. The line wings are brighter (relative to the line center intensity) in HCN than in CO 1-0 and line ratios suggest that the molecular outflow consists of dense (n > 104 cm-3) and clumpy gas with a high HCN abundance X(HCN) > 10-8. These properties are consistent with the molecular gas being compressed and fragmented by shocks in the outflow. Alternatively, HCN is instead pumped by mid-IR continuum, but we propose that this effect is not strong for the spatially extended outflowing gas. In addition, we find that the rotation of the main disk, in east-west direction, is also evident in the HCN, HCO+ and HNC line emission. An unexpectedly bright HC3N 10-9 line is detected inside the central 400 pc of Mrk 231. This HC3N emission may emerge from a shielded, dust-enshrouded region within the inner 40-50 pc where the gas is heated to high temperatures (200-300 K) by the AGN. © 2012 ESO.