The Deep Silicate Absorption Feature in IRAS 08572+3915 and Other Infrared Galaxies

New mid-infrared (10 and 20 μm) spectro-photometry of the ultraluminous infrared galaxy IRAS 08572+3915 is presented. The 10 μm spectrum reveals a deep silicate absorption feature, while the 20 μm spectrum shows no clear evidence for an 18 μm silicate absorption feature. An interstellar extinction curve is fitted to IRAS 08572+3915 and two other deep silicate infrared galaxies, NGC 4418 and Arp 220. It is found that pure extinction cannot explain the spectral energy distributions of these sources. On the other hand, both the strength of the silicate absorption and the overall spectral energy distributions of the three galaxies agree well with scaled-up models of galactic protostars. From this agreement, we conclude that the infrared emission comes from an optically thick dust shell surrounding a compact power source. The size of the power source is constrained to be smaller than a few parsecs. We argue that a significant portion of the total luminosities of these galaxies arises from an active galactic nucleus deeply embedded in dust. Subject headings: dust — galaxies: individual: IRAS 08572+3915 — NGC 4418 — Arp 220 — galaxies: active — galaxies: nuclei — infrared: galaxies Preprint IfA-97-31 aka astro-ph/9705242: To Appear in The Astrophysical Journal. Visiting Astronomer at the United Kingdom Infrared Telescope, which is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. Visiting Astronomer at the NASA Infrared Telescope Facility, which is operated by the University of Hawaii under contract to NASA.