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 NGC 4418, 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 μm 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 μm, 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 ∼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.