Evidence for a chemically differentiated outflow in Mrk 231^⋆,⋆⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
e-mail: johan.lindberg@nasa.gov
² NASA Goddard Space Flight Center, Astrochemistry Laboratory, Mail Code 691, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
³ Osservatorio di Radioastronomia (ORA-INAF), Italian ALMA Regional Centre, c/o CNR, via Gobetti 101, 40129 Bologna, Italy
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁸ Universidad de Alcalá de Henares, Departamento de Física, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain

Received: 27 September 2015
Accepted: 14 December 2015

Abstract

Aims. Our goal is to study the chemical composition of the outflows of active galactic nuclei and starburst galaxies.

Methods. We obtained high-resolution interferometric observations of HCN and HCO⁺J = 1 → 0 and J = 2 → 1 of the ultra-luminous infrared galaxy Mrk 231 with the IRAM Plateau de Bure Interferometer. We also use previously published observations of HCN and HCO⁺J = 1 → 0 and J = 3 → 2, and HNC J = 1 → 0 in the same source.

Results. In the line wings of the HCN, HCO⁺, and HNC emission, we find that these three molecular species exhibit features at distinct velocities which differ between the species. The features are not consistent with emission lines of other molecular species. Through radiative transfer modelling of the HCN and HCO⁺ outflow emission we find an average abundance ratio X(HCN) /X(HCO⁺) ≳ 1000. Assuming a clumpy outflow, modelling of the HCN and HCO⁺ emission produces strongly inconsistent outflow masses.

Conclusions. Both the anti-correlated outflow features of HCN and HCO⁺ and the different outflow masses calculated from the radiative transfer models of the HCN and HCO⁺ emission suggest that the outflow is chemically differentiated. The separation between HCN and HCO⁺ could be an indicator of shock fronts present in the outflow, since the HCN/HCO⁺ ratio is expected to be elevated in shocked regions. Our result shows that studies of the chemistry in large-scale galactic outflows can be used to better understand the physical properties of these outflows and their effects on the interstellar medium in the galaxy.