Vibrationally excited HCN in the luminous infrared galaxy NGC 4418

Abstract

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 d tection of (sub)millimeter otational lines of vibrationally excited HCN, in the dust-enshrouded nucleus of the luminous infrared galaxy GC 4418. We estimate the excitation temperature of Tvib ≈ 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, N 2H+, and HC3N at λ ∼ 1 mm. Their relative intensities may also be affected by the infrared pumping. 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.