Vibrationally Excited HCN in the Luminous Infrared Galaxy NGC 4418

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 λ ~ 1 mm. Their relative intensities may also be affected by the infrared pumping.