Enhanced UV radiation and dense clumps in the molecular outflow of Mrk 231

Abstract

We present interferometric observations of the CN(1-0) line emission in Mrk 231 and combine them with previous observations of CO and other H 2 gas tracers to study the physical properties of the massive molecular outflow. We find a strong boost of the CN/CO(1-0) line luminosity ratio in the outflow, which is unprecedented compared to any other known Galactic or extragalactic source. For the dense gas phase in the outflow traced by the HCN and CN emissions, we infer X CN ≡ [CN]/[H 2 ] > X HCN by at least a factor of three, with H 2 gas densities of n H 2 ∼ 10 5−6 cm −3 . In addition, for the first time, we resolve narrow spectral features in the HCN(1-0) and HCO + (1-0) high-velocity line wings tracing the dense phase of the outflow. The velocity dispersions of these spectral features, σ v ∼ 7 − 20 km s −1 , are consistent with those of massive extragalactic giant molecular clouds detected in nearby starburst nuclei. The H 2 gas masses inferred from the HCN data are quite high, M mol ∼ 0.3 − 5 × 10 8 M . Our results suggest that massive, denser molecular gas complexes survive embedded into the more diffuse H 2 phase of the outflow, and that the chemistry of such outflowing dense clouds is affected by enhanced UV radiation.

Cool outflows in galaxies and their implications

“…CN has been detected in the double outflows of the merger NGC 3256 (Sakamoto et al 2014) and more studies are required to understand its origin. Very luminous CN emission is also detected in the quasar-driven molecular outflow of Mrk 231 (Cicone et al 2019) where X(CN) > X(HCN), suggesting strong radiative impact on the outflowing dense molecular gas. C 2 H has been found in the starburst-driven outflow of Maffei 2 (Meier and Turner 2012) and also in the interface region between the molecular disk and ionized gas outflow of NGC 1068 (García-Burillo et al 2017).…”

“…A recent study reveal very bright CN emission from the Mrk 231 outflow. This may be due to UV photons originating from star formation in the outflow itself, or from the quasar (Cicone et al 2019). González-Alfonso et al ( 2018) have recently argued that the high OH + abundances relative to OH, H 2 O, and H 3 O + measured from the Herschel spectra imply a high ionization rate that cannot be explained by the unattenuated hard X-ray flux measured in this object (Teng et al 2014).…”

“…Some of these observations are very detailed and reveal very clumpy structures (e.g. Pereira-Santaella et al 2016Cicone et al 2019) and complex morphologies on different scales, often difficult to describe with simple models (Combes et al 2014;Tsai et al 2012;Sakamoto et al 2014;García-Burillo et al 2014Gallimore et al 2016;Aalto et al 2016;Pereira-Santaella et al 2016;Falstad et al 2017;Pereira-Santaella et al 2018;Herrera-Camus et al 2019a;Treister et al 2020). However, the bulk of these observations are often still limited to the detection of high-velocity wings of the CO transitions, and only marginally resolved.…”

Super

“…Krug et al 2010;Rupke & Veilleux 2011;Cazzoli et al 2016;Concas et al 2019;Roberts-Borsani 2020), cold molecular gas phase, using different transitions of CO, HCN and [C ii], for instance (e.g. García-Burillo et al 2014;Tadhunter et al 2014;Feruglio et al 2017;Aladro et al 2018;Michiyama et al 2018;Zschaechner et al 2018;Aalto et al 2019;Husemann et al 2019;Cicone et al 2020;Veilleux et al 2020), warm and hot molecular gas phase, using transitions in the mid-to near-infrared (Veilleux et al 2009;Davies et al 2014;Hill & Zakamska 2014;Riffel et al 2015;Emonts et al 2017;May et al 2018;Petric et al 2018;Riffel et al 2020) and ionised gas phase observed using the rest-frame optical emission lines such as the forbidden transitions of [O iii] λ5007 (e.g. Harrison et al 2014;Kakkad et al 2016;Zakamska et al 2016;Fiore et al 2017;Venturi et al 2018;Baron & Netzer 2019;Coatman et al 2019;Förster Schreiber et al 2019).…”

MUSE view of Arp220: Kpc-scale multi-phase outflow and evidence for positive feedback

“…High-resolution and multi-wavelength observations are therefore required to study outflows in detail (e.g. Emonts et al 2017;Pereira-Santaella et al 2018;Cicone et al 2020).…”