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We present the results of ALMA Cycle 2 observations of the ultraluminousinfrared galaxy, IRAS 20551-4250, at HCN/HCO+/HNC J=3-2 lines at bothvibrational-ground (v=0) and vibrationally excited (v2=1) levels. This galaxycontains a luminous buried active galactic nucleus (AGN), in addition tostarburst activity, and our ALMA Cycle 0 data revealed a tentatively detectedvibrationally 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 andHNC v2=1f J=3-2 emission lines are also detected, but the HCO+ v2=1f J=3-2emission line is not. Given the high-energy level of v2=1 and the resultingdifficulty of collisional excitation, we compared these results with those ofthe calculation of infrared radiative pumping, using the available infrared5-35 micron spectrum. We found that all of the observational results werereproduced, if the HCN abundance was significantly higher than that of HCO+ andHNC. The flux ratio and excitation temperature between v2=1f and v=0, aftercorrection for possible line opacity, suggests that infrared radiative pumpingaffects rotational (J-level) excitation at v=0 at least for HCN and HNC. TheHCN-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. Theenhanced HCN-to-HCO+ flux ratio in this AGN-hosting galaxy can be explained bythe high HCN-to-HCO+ abundance ratio and sufficient HCN excitation at up toJ=4, rather than the significantly higher efficiency of infrared radiativepumping for HCN than HCO+.

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