2017ApJ...849...29I

We present our ALMA multi-transition molecular line observational results for the ultraluminous infrared galaxy IRAS 20551-4250, which is known to contain a luminous buried active galactic nucleus and shows detectable vibrationally excited (v₂ = 1f) HCN and HNC emission lines. The rotational J = 1-0, 4-3, and 8-7 of HCN, HCO⁺, and HNC emission lines were clearly detected at a vibrational ground level (v = 0). Vibrationally excited (v₂ = 1f) J = 4-3 emission lines were detected for HCN and HNC, but not for HCO⁺. Their observed flux ratios further support our previously obtained suggestion, based on J = 3-2 data, that (1) infrared radiative pumping plays a role in rotational excitation at v = 0, at least for HCN and HNC, and (2) HCN abundance is higher than HCO⁺ and HNC. The flux measurements of the isotopologue H¹³CN, H¹³CO⁺, and HN¹³C J = 3-2 emission lines support the higher HCN abundance scenario. Based on modeling with collisional excitation, we constrain the physical properties of these line-emitting molecular gases, but find that higher HNC rotational excitation than HCN and HCO⁺ is difficult to explain, due to the higher effective critical density of HNC. We consider the effects of infrared radiative pumping using the available 5-30 µm infrared spectrum and find that our observational results are well-explained if the radiation source is located at 30-100 pc from the molecular gas. The simultaneously covered very bright CO J = 3-2 emission line displays a broad emission wing, which we interpret as being due to molecular outflow activity with the estimated rate of ∼150 M_☉ yr^–1.