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

Abstract

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 μm 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+.