Vibrationally Excited HCN in the Luminous Infrared Galaxy NGC 4418

TL;DR

This study reports the first detection of vibrationally excited HCN in an extragalactic source, revealing infrared pumping effects on molecular excitation in the nucleus of NGC 4418, a luminous infrared galaxy.

Contribution

It provides the first extragalactic detection of vibrationally excited HCN and estimates its excitation temperature, highlighting infrared radiation's role in molecular excitation in galaxy nuclei.

Findings

Detected vibrationally excited HCN in NGC 4418

Estimated vibrational excitation temperature ~230 K

Infrared radiation influences molecular excitation levels

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 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.