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

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 HC 3 N 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 blueand 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 > 10 4 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 HC 3 N 10-9 line is detected inside the central 400 pc of Mrk 231. This HC 3 N 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.

The Mrk 231 molecular outflow as seen in OH

“…If the receding OH is behind the source of 119 μm continuum emission, only ∼20% of that continuum within Δv = 500 km s −1 is thus intercepted and reemitted by the OH, indicating high clumpiness of the spatially extended reemitting OH. This is consistent with the clumpiness of the outflow inferred by Aalto et al (2012) from HCN emission. The ground-state OH79 doublet shows a relatively strong emission feature, with a flux that is nearly 75% of the absorption flux.…”

“…We note, however, that this solution relies on our simple spherical geometry (where the successive shells are concentric) and may not be unique; for example, the high-and low-velocity gas may be flowing from different regions of a circumnuclear torus or disk, characterized by different T dust and possibly with different projection effects as the outflow widens. Nevertheless, some deceleration is most likely taking place because CO and HCN, which trace larger regions, show wings up to a velocity of ∼800 km s −1 from the line center (Feruglio et al 2010;Cicone et al 2012;Aalto et al 2012), significantly lower than OH. Spoon & Holt (2009) also inferred a decelerating velocity field from the ionized gas outflows traced by the [NeII], [NeIII] and [NeV] lines in a sample of ULIRGs, though not in Mrk 231; the fine-structure mid-IR lines trace an outflow on a significantly smaller spatial scale, however.…”

High-resolution imaging of the molecular outflows in two mergers: IRAS 17208-0014 and NGC 1614

“…The outflow phenomenon concerns virtually all the phases of the interstellar medium (ISM), including the ionized gas (e.g., Heckman et al 1990;Colina et al 1991;Martin 1998;Holt et al 2008;Crenshaw et al 2010;Rupke & Veilleux 2013;Arribas et al 2014), the neutral A&A 580, A35 (2015) atomic medium (e.g., Heckman et al 2000;Rupke et al 2002Rupke et al , 2005aRupke et al , 2005bRupke et al , 2005cMorganti et al 2005;Martin 2005Martin , 2006Chen et al 2010), as well as the molecular gas phase (e.g., Feruglio et al 2010;Fischer et al 2010;Sturm et al 2011;Alatalo et al 2011Alatalo et al , 2015Chung et al 2011;Aalto et al 2012;Dasyra & Combes 2012;Maiolino et al 2012;Combes et al 2013;Morganti et al 2013;Veilleux et al 2013;Cicone et al 2012Cicone et al , 2014Cicone et al , 2015García-Burillo et al 2014;González-Alfonso et al 2014;Davies et al 2014;Emonts et al 2014;Sakamoto et al 2014). This multiwavelength evidence underlines that the overall impact of outflows can only be quantified if all of their ISM phases are studied in different populations of galaxies.…”

Feeding versus feedback in AGN from near-infrared IFU observations XI: NGC 2110

“…On the other hand, recent studies have also found massive outflows of molecular gas (Sakamoto et al 2010;Aalto et al 2012;Veilleux et al 2013), in particular, in Luminous Infrared Galaxies (LIRGS) and Ultra Luminous Infrared Galaxies (ULIRGS).…”