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