2016ApJ...825...44I

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 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 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 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⁺.