Enhanced UV radiation and dense clumps in Mrk231's molecular outflow

TL;DR

This study uses interferometric observations to analyze the molecular outflow in Mrk231, revealing unprecedented line ratios, dense gas properties, and spectral features that shed light on the outflow's physical and chemical conditions.

Contribution

It provides the first resolved spectral features of dense gas tracers in Mrk231's outflow and highlights the impact of UV radiation on outflow chemistry.

Findings

Unprecedented CN/CO luminosity ratio boost in the outflow

Dense gas traced by HCN and CN has densities of 10^{5-6} cm^{-3}

High H2 gas masses in the outflow, up to 5×10^8 solar masses

Abstract

We present interferometric observations of the CN(1-0) line emission in Mrk231 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 $\rm X_{\rm CN}\equiv [CN]/[H_2] > X_{\rm HCN}$ by at least a factor of three, with H$_2$ gas densities of $n_{\rm H_2}\sim10^{5-6}$ 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, $\sigma_v\sim7-20$ 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, $M_{mol}\sim0.3-5\times10^8$ $M_{\odot}$. 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.