HCN (1-0) opacity of outflowing gas in Arp 220W

TL;DR

This study uses high-resolution ALMA data to analyze the HCN/H13CN 1-0 line ratio in Arp 220W's outflow, revealing low opacity of HCN 1-0 and implications for molecular gas mass estimates in galactic outflows.

Contribution

It provides the first constraint on the HCN/H13CN 1-0 line ratio in Arp 220W's outflow, indicating low optical depth and suggesting a revised, lower conversion factor for dense gas mass.

Findings

HCN/H13CN 1-0 line ratio is at least three times higher in the outflow than in the entire system.

High line ratio suggests low opacity of HCN 1-0 in the outflow.

Lower conversion factor for HCN 1-0 luminosity should be used for outflowing gas.

Abstract

Context. We present our findings for the HCN/H13CN 1-0 line ratio in the molecular outflow of Arp 220 west based on high-resolution ALMA data. Aims. Molecular gas masses in the outflowing gas of galaxies driven by active galactic nuclei (AGNs) or starbursts are important parameters for understanding the feedback of these latter two phenomena and star-formation quenching. The conversion factor of line luminosities to masses is related to the optical depth of the molecular lines. Methods. Using H13CN 1-0, the isotopic line of HCN 1-0, to obtain the line ratio of HCN/H13CN 1-0 is an ideal way to derive the optical depth of HCN 1-0 in outflowing gas. Results. With the nondetection of H13CN 1-0 in the outflowing gas, a 3{\sigma} lower limit of HCN/H13CN 1-0 line ratio is obtained, which is at least three times higher than that found in the whole of the whole system of Arp 220. The high HCN/H13CN 1-0 line ratio indicates low opacity of HCN 1-0 in the outflowing gas, even though the upper limit of HCN 1-0 opacity obtained here is still not good enough to draw any robust conclusions if HCN 1-0 is optically thin. A lower conversion factor of HCN 1-0 luminosity to dense gas mass in the outflowing gas should be used than that used for the host galaxy of Arp 220.