The compact nucleus of the deep silicate absorption galaxy NGC 4418

High-resolution Hubble Space Telescope (HST) near-infrared and Keck mid-infrared images of the heavily extinguished infrared-luminous galaxy NGC 4418 are presented. These data make it possible to observe the imbedded near-infrared structure on scales of 10-20 pc and to constrain the size of the mid-infrared-emitting region. The 1.1-2.2 μm data of NGC 4418 show no clear evidence of nuclear star clusters or of a reddened active galactic nucleus. Instead, the nucleus of the galaxy consists of a ∼100-200 pc linear structure with fainter structures extending radially outward. The near-infrared colors of the linear feature are consistent with a 10-300 Myr starburst suffering moderate levels (a few magnitudes) of visual extinction. At 7.9-24.5 μm NGC 4418 has estimated size upper limits in the range of 30-80 pc. These dimensions are consistent with the highest-resolution radio observations obtained to date of NGC 4418, as well as the size of 50-70 pc expected for a blackbody with a temperature derived from the 25, 60, and 100 μm flux densities of the galaxy. Further, a spectral energy distribution constructed from the multiwavelength mid-infrared observations shows the strong silicate absorption feature at 10 μm, consistent with previous mid-infrared observations of NGC 4418. An infrared surface brightness of ∼2.1 × 10¹³ L_⊙ kpc^-2 is derived for NGC 4418. Such a value, though consistent with the surface brightness of warm ultraluminous infrared galaxies [L_IR (8-1000 μm) ≥ 10 ¹² L_⊙], such as IRAS 05189-2524 and IRAS 08572+3915, is not large enough to distinguish NGC 4418 as a galaxy powered by an active galactic nucleus, as opposed to a lower surface brightness starburst.