Near-infrared and millimeter constraints on the nuclear energy source of the infrared-luminous galaxy NGC 4418

We present near-infrared and millimeter investigations of the nucleus of the infrared-luminous galaxy NGC4418, which previous observations suggest possesses a powerful buried active galactic nucleus (AGN). We find the following main results: ( 1) The infrared K-band spectrum shows CO absorption features at 2.3 - 2.4 mum from stars and very strong H-2 emission lines. The luminosity ratios of H-2 emission lines are suggestive of a thermal origin, and the equivalent width of the H-2 1 - 0 S(1) line is the second largest observed to date in an external galaxy, after the well-studied strong H-2-emitting galaxy NGC 6240. ( 2) The infrared L-band spectrum shows a clear polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 mum, which is usually found in star-forming galaxies. The estimated star formation luminosity from the observed PAH emission can account for only a small fraction of the infrared luminosity. ( 3) Millimeter interferometric observations of the nucleus reveal a high HCN ( 1 - 0) to HCO+ ( 1 - 0) luminosity ratio of similar to 1.8, as has been previously found in pure AGNs. ( 4) The measurements of HCN ( 1 - 0) luminosity using a single-dish millimeter telescope show that the HCN ( 1 - 0) to infrared luminosity ratio is slightly larger than the average, but within the scattered range, for other infrared-luminous galaxies. All of these results can be explained by the scenario in which, in addition to energetically insignificant, weakly obscured star formation at the surface of the nucleus, a powerful X-ray - emitting AGN deeply buried in dust and high-density molecular gas is present.