ALMA Investigation of Vibrationally Excited HCN/HCO+/HNC Emission Lines in the AGN-Hosting Ultraluminous Infrared Galaxy IRAS 20551-4250

TL;DR

This study uses ALMA observations to analyze vibrationally excited HCN, HCO+, and HNC emission lines in the IRAS 20551-4250 galaxy, revealing insights into molecular excitation mechanisms influenced by an embedded AGN.

Contribution

It provides the first detailed comparison of vibrationally excited and ground state emission lines in an AGN-hosting ULIRG, highlighting the role of infrared radiative pumping and molecular abundance ratios.

Findings

HCN and HNC vibrationally excited lines detected; HCO+ not detected.

Infrared radiative pumping explains excitation levels when HCN abundance is high.

Enhanced HCN-to-HCO+ flux ratio indicates high HCN abundance and excitation in the galaxy.

Abstract

We present the results of ALMA Cycle 2 observations of the ultraluminous infrared galaxy, IRAS 20551-4250, at HCN/HCO+/HNC J=3-2 lines at both vibrational-ground (v=0) and vibrationally excited (v2=1) levels. This galaxy contains a luminous buried active galactic nucleus (AGN), in addition to starburst activity, and our ALMA Cycle 0 data revealed a tentatively detected vibrationally excited HCN v2=1f J=4-3 emission line. In our ALMA Cycle 2 data, the HCN/HCO+/HNC J=3-2 emission lines at v=0 are clearly detected. The HCN and HNC v2=1f J=3-2 emission lines are also detected, but the HCO+ v2=1f J=3-2 emission line is not. Given the high-energy level of v2=1 and the resulting difficulty of collisional excitation, we compared these results with those of the calculation of infrared radiative pumping, using the available infrared 5-35 micron spectrum. We found that all of the observational results were reproduced, if the HCN abundance was significantly higher than that of HCO+ and HNC. The flux ratio and excitation temperature between v2=1f and v=0, after correction for possible line opacity, suggests that infrared radiative pumping affects rotational (J-level) excitation at v=0 at least for HCN and HNC. The HCN-to-HCO+ v=0 flux ratio is higher than those of starburst-dominated regions, and will increase even more when thederived high HCN opacity is corrected. The enhanced HCN-to-HCO+ flux ratio in this AGN-hosting galaxy can be explained by the high HCN-to-HCO+ abundance ratio and sufficient HCN excitation at up to J=4, rather than the significantly higher efficiency of infrared radiative pumping for HCN than HCO+.