ALMA INVESTIGATION OF VIBRATIONALLY EXCITED HCN/HCO + /HNC EMISSION LINES IN THE AGN-HOSTING ULTRALUMINOUS INFRARED GALAXY IRAS 20551−4250

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 ( v 2 = 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 v 2 = 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 v 2 = 1f J = 3–2 emission lines are also detected, but the HCO + v 2 = 1f J = 3–2 emission line is not. Given the high energy level of v 2 = 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 μ m 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 v 2 = 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 the derived 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 + .