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