  New mid-infrared (10 and 20 micron) spectro-photometry of the ultraluminous
infrared galaxy IRAS 08572+3915 is presented. The 10 micron spectrum reveals a
deep silicate absorption feature, while the 20 micron spectrum shows no clear
evidence for an 18 micron 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.
