The Compact Nucleus of the Deep Silicate Absorption Galaxy NGC 4418

High resolution, Hubble Space Telescope (HST) near-infrared and Keckmid-infrared images of the heavily extinguished, infrared luminous galaxy NGC4418 are presented. These data make it possible to observe the imbeddednear-infrared structure on scales of 10-20 pc, and to constrain the size of themid-infrared emitting region. The 1.1-2.2 um data of NGC 4418 show no clearevidence of nuclear star clusters or of a reddened active galactic nucleus.Instead, the nucleus of the galaxy consists of a ~100-200 pc linear structurewith fainter structures extending radially outward. The near-infrared colors ofthe linear feature are consistent with a 10-300 Myr starburst sufferingmoderate levels (few magnitudes) of visual extinction. At 7.9-24.5 um, NGC 4418has estimated size upper limits in the range of 30-80 pc. These dimensions areconsistent with the highest resolution radio observations obtained to date ofNGC 4418, as well as the size of 50-70 pc expected for a blackbody with atemperature derived from the 25 um, 60 um, and 100 um flux densities of thegalaxy. Further, a spectral energy distribution constructed from themulti-wavelength mid-infrared observations show the strong silicate absorptionfeature at 10 um, consistent with previous mid-infrared observations of NGC4418. An infrared surface brightness of 2.1x10^13 L_sun kpc^-2 is derived forNGC 4418. Such a value, though consistent with the surface brightness of warmultraluminous infrared galaxies (ULIGs: L_IR [8-1000 um] >~ 10^12 L_sun) suchas IRAS 05189-2524 and IRAS 08572+3915, is not large enough to distinguish NGC4418 as a galaxy powered by an Active Galactic Nucleus (AGN), as opposed to alower surface brightness starburst.Comment: LaTex, 7 pages, including 2 jpg figures and 3 postscript figures, AJ, in press (May, 2003