Detection of HCN, HCO⁺, and HNC in the Mrk 231 molecular outflow^⋆

Dense molecular gas in the AGN wind

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
e-mail: saalto@chalmers.se
² Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 - Madrid, Spain
³ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St. Martin d′Hères, France
⁴ Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Astronomy Department, King Abdulaziz University, PO Box 80203 Jeddah 21589, Saudi Arabia

Received: 19 August 2011
Accepted: 14 November 2011

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.