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

New mid-infrared (10 and 20 micron) spectro-photometry of the ultraluminousinfrared galaxy IRAS 08572+3915 is presented. The 10 micron spectrum reveals adeep silicate absorption feature, while the 20 micron spectrum shows no clearevidence for an 18 micron silicate absorption feature. An interstellarextinction curve is fitted to IRAS 08572+3915 and two other deep silicateinfrared galaxies, NGC 4418 and Arp 220. It is found that pure extinctioncannot explain the spectral energy distributions of these sources. On the otherhand, both the strength of the silicate absorption and the overall spectralenergy distributions of the three galaxies agree well with scaled-up models ofgalactic protostars. From this agreement, we conclude that the infraredemission comes from an optically thick dust shell surrounding a compact powersource. The size of the power source is constrained to be smaller than a fewparsecs. We argue that a significant portion of the total luminosities of thesegalaxies arises from an active galactic nucleus deeply embedded in dust.Comment: 25 pages with 5 embedded figures in AASLaTeX4.0 preprint format. To be published in The Astrophysical Journa