The Compact Nucleus of the Deep Silicate Absorption Galaxy NGC 4418

Abstract

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 × 10^(13) 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.