Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2010-11-30
Astrophys.J.725:L228-L233,2010
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
ApJL in press, 8 pages
Scientific paper
10.1088/2041-8205/725/2/L228
Infrared pumping and its effect on the excitation of HCN molecules can be important when using rotational lines of HCN to probe dense molecular gas in galaxy nuclei. We report the first extragalactic detection of (sub)millimeter rotational lines of vibrationally excited HCN, in the dust-enshrouded nucleus of the luminous infrared galaxy NGC 4418. We estimate the excitation temperature of T_vib ~ 230 K between the vibrational ground and excited (v_2=1) states. This excitation is most likely due to infrared radiation. At this high vibrational temperature the path through the v_2=1 state must have a strong impact on the rotational excitation in the vibrational ground level, although it may not be dominant for all rotational levels. Our observations also revealed nearly confusion limited lines of CO, HCN, HCO+, H13CN, HC15N, CS, N2H+, and HC3N at lambda ~ 1 mm. Their relative intensities may also be affected by the infrared pumping.
Aalto Susanne
Evans Aaron S.
Sakamoto Kazushi
Wiedner Martina C.
Wilner David J.
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