High resolution observations of HCN and HCO⁺J = 3–2 in the disk and outflow of Mrk 231

Detection of vibrationally excited HCN in the warped nucleus^⋆,⋆⋆

¹ Department of Earth and Space SciencesChalmers 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, 38406St. Martin d′Hères, France
⁴ Universidad de Alcalá de Henares, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
⁵ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
⁶ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía, s/n, 18008 Granada, Spain
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia

Received: 14 April 2014
Accepted: 3 November 2014

Abstract

Aims. Our goal is to study molecular gas properties in nuclei and large scale outflows/winds from active galactic nuclei (AGNs) and starburst galaxies.

Methods. We obtained high resolution (0.̋25 to 0.̋90) observations of HCN and HCO⁺J = 3 → 2 of the ultraluminous QSO galaxy Mrk 231 with the IRAM Plateau de Bure Interferometer (PdBI).

Results. We find luminous HCN and HCO⁺J = 3 → 2 emission in the main disk and we detect compact (r ≲ 0''̣1 (90 pc)) vibrationally excited HCN J = 3 → 2ν₂ = 1f emission centred on the nucleus. The velocity field of the vibrationally excited HCN is strongly inclined (position angle PA = 155°) compared to the east-west rotation of the main disk. The nuclear (r ≲ 0.̋1) molecular mass is estimated to 8 × 10⁸ M_⊙ with an average N(H₂) of 1.2 × 10²⁴ cm^-2. Prominent, spatially extended (≳350 pc) line wings are found for HCN J = 3 → 2 with velocities up to ± 750 km s^-1. Line ratios indicate that the emission is emerging in dense gas n = 10⁴−5 × 10⁵ cm^-3 of elevated HCN abundance X(HCN) = 10^-8−10^-6. The highest X(HCN) also allows for the emission to originate in gas of more moderate density. We tentatively detect nuclear emission from the reactive ion HOC⁺ with HCO⁺/HOC⁺ = 10−20.

Conclusions. The HCN ν₂ = 1f line emission is consistent with the notion of a hot, dusty, warped inner disk of Mrk 231 where the ν₂ = 1f line is excited by bright mid-IR 14 μm continuum. We estimate the vibrational temperature T_vib to 200−400 K. Based on relative source sizes we propose that 50% of the main HCN emission may have its excitation affected by the radiation field through IR pumping of the vibrational ground state. The HCN emission in the line wings, however, is more extended and thus likely not strongly affected by IR pumping. Our results reveal that dense clouds survive (and/or are formed) in the AGN outflow on scales of at least several hundred pc before evaporating or collapsing. The elevated HCN abundance in the outflow is consistent with warm chemistry possibly related to shocks and/or X-ray irradiated gas. An upper limit to the mass and momentum flux is 4 × 10⁸ M_⊙ and 12L_AGN/c, respectively, and we discuss possible driving mechanisms for the dense outflow.