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

Abstract

New mid-infrared (10 and 20 km) spectrophotometry of the ultraluminous infrared galaxy IRAS 08572]3915 is presented. The 10 km spectrum reveals a deep silicate absorption feature, while the 20 km spectrum shows no clear evidence for an 18 km silicate absorption feature. An interstellar extinction curve is Ðtted 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 signiÐcant portion of the total luminosities of these galaxies arises from an active galactic nucleus deeply embedded in dust.

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

“…For the most extreme high-U cases, such as might be present in buried or partially obscured AGNs, the dust emission at the dust sublimation radius must peak at near-IR wavelengths (Dudley & Wynn-Williams 1997). In this case, radiative transfer effects could produce excess FIR emission arising from a central source that is not associated with the normal PDR tracers in these galaxies and may suffer higher extinction than the PDR components.…”

“…As a precaution, we have plotted as open symbols sources that are listed as resolved at 25 or 60 lm or both, although we have made use of the LWS data to estimate the FIR flux where appropriate in the present sample. It is of interest that for the strongest upper limit in this plot (NGC 4418), the power source of the FIR is considered to be a buried AGN based on its mid-IR spectral properties (Roche et al 1986;Dudley & Wynn-Williams 1997;Spoon et al 2001). The data in Figure 9 suggest a break occurring near 10 12 L , although additional data, say with the Herschel and Astro-F space telescopes, are required to rule out a continuing decline at higher luminosities and to better define the sharpness of the break.…”

“…In this scenario, migration of the break in Figure 9 may not be observable at submillimeter wavelengths. A more detailed discussion of the cosmological implications of the break in the [C ii] emission around 10 12 L in light of the above scenarios can be found in Dudley et al (2003).…”

Infrared Photometric Study for theIRAS1 Jy Sample of Ultraluminous Infrared Galaxies

“…Carico et al (1990) and Dudley & Wynn-Williams (1997) also obtained a larger H À K color. Dudley & Wynn-Williams (1997) made observations in the 8-24 m region, and the result shows that there is strong silicate absorption centered at 9.7 m, indicating infrared emission from an optically thick dust shell surrounding a compact power source.…”

“…Carico et al (1990) and Dudley & Wynn-Williams (1997) also obtained a larger H À K color. Dudley & Wynn-Williams (1997) made observations in the 8-24 m region, and the result shows that there is strong silicate absorption centered at 9.7 m, indicating infrared emission from an optically thick dust shell surrounding a compact power source. In addition, no polycyclic aromatic hydrocarbon emission is detected at 3.3 m, but a very strong 3.4 m carbonaceous dust absorption feature is displayed for F08572+3915 that indicates the source being powered by highly obscured AGN activity (Imanishi & Ueno 2000).…”