Astrophys. J., 849, 29-29 (2017/November-8)
ALMA multiple-transition molecular line observations of the ultraluminous infrared galaxy IRAS 20551-4250: different HCN, HCO+, and HNC excitation, and implications for infrared radiative pumping.
IMANISHI M., NAKANISHI K. and IZUMI T.
Abstract (from CDS):
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.
© 2017. The American Astronomical Society. All rights reserved.
galaxies: active - galaxies: nuclei - galaxies: Seyfert - galaxies: starburst - quasars: general - submillimeter: galaxies - submillimeter: galaxies
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