Mid-Infrared Silicate Features as an Indicator of the Dust Density Distribution in ULIRGs

Details Mid-Infrared Silicate Features as an Indicator of the Dust Density Distribution in ULIRGs Mid-Infrared Silicate Features as an Indicator of the Dust Density Distribution in ULIRGs

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20914906s&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #149.06; Bulletin of the American Astronomical Society,

Astronomy and Astrophysics

Astronomy

Scientific paper

The central heating sources in ultraluminous infrared galaxies (ULIRGs) are surrounded by significant amounts of material. Can we determine the dust density distribution of this material? Dust reprocesses the x-ray and ultraviolet radiation of the central source and reemits this radiation in the mid-infrared (MIR) and far-infrared (FIR). Silicate dust has two prominent features in the MIR near 10μm and 18.0μm. ULIRGs show these two spectral features in absorption. There must be a thermal gradient to produce an absorption feature. The depth of the silicate features depends upon the thermal gradient, and the thermal gradient depends upon the dust density distribution. Therefore, the silicate feature depths are directly related to the dust density distribution.
Using the numerical radiative transfer code DUSTY, we model different dust distributions: a simple normally-illuminated slab, a continuous density shell, and a clumpy shell. For a given density structure with optical depths that produce both features in absorption, we find that the ratio of the 10μm and the 18μm depths is generally constant. Departures from this linear relationship are a consequence of variations to the thermal gradient. The relationship between the 10μm and 18μm feature depths, therefore, directly indicates the dust density distribution. Consequently, we find the central heating source spectral shape, be it starburst or active galactic nuclei, does not influence the MIR and FIR spectra with high obscuration.
MMS and NAL acknowledge work supported by the National Science Foundation under Grant No. 0237291.

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