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摘要
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 mu m 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+.
- 出版日期2016-7-1
- 单位中国科学院国家天文台
