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
Abstract
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 (v 2 = 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 (v 2 = 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, {{{H}}}13{CO}}+, 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2017
- DOI:
- 10.3847/1538-4357/aa7ff9
- arXiv:
- arXiv:1708.08073
- Bibcode:
- 2017ApJ...849...29I
- Keywords:
-
- galaxies: active;
- galaxies: nuclei;
- galaxies: Seyfert;
- galaxies: starburst;
- quasars: general;
- submillimeter: galaxies;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 37 pages, 19 figures, accepted for publication in ApJ