ALMA Detection of the Vibrationally Excited HCN J=4-3 Emission Line in the AGN-Hosting Luminous Infrared Galaxy IRAS 20551-4250

TL;DR

This study reports the first tentative detection of vibrationally excited HCN J=4-3 emission in an AGN-hosting galaxy, IRAS 20551-4250, using ALMA, indicating vibrational excitation may be common in such environments.

Contribution

First detection of vibrationally excited HCN J=4-3 in an external galaxy, supporting the role of infrared radiative pumping in AGN environments.

Findings

Vibrationally excited HCN detected at ~5 sigma significance.

Higher HCN-to-HCO+ flux ratio compared to starburst galaxies.

Small molecular line widths suggest vibrational excitation is common in AGN hosts.

Abstract

We present results from our ALMA Cycle 0 observations, at the frequencies around the HCN, HCO+, and HNC J=4-3 transition lines, of the luminous infrared galaxy IRAS 20551-4250 at z=0.043, which is known to host an energetically important obscured AGN. In addition to the targeted HCN, HCO+, and HNC J=4-3 emission lines, two additional strong emission lines are seen, which we attribute to H2S and CH3CN(+CCH). The HCN-to-HCO+ J=4-3 flux ratio (~0.7) is higher than in the other starburst-dominated galaxy (~0.2) observed in our ALMA Cycle 0 program. We tentatively (~5 sigma) detected the vibrationally excited (v2=1) HCN J=4-3 (l=1f) emission line, which is important for testing an infrared radiative pumping scenario for HCN. This is the second detection of this molecular transition in external galaxies. The most likely reason for this detection is not only the high flux of this emission line, but also the small molecular line widths observed in this galaxy, suggesting that vibrational excitation of HCN may be relatively common in AGN-hosting galaxies.