Exploring the nature of the 9-13 micron silicate emission discovered in quasars

Details Exploring the nature of the 9-13 micron silicate emission discovered in quasars Exploring the nature of the 9-13 micron silicate emission discovered in quasars

Jun 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005sptz.prop20231s&link_type=abstract

Spitzer Proposal ID #20231

Physics

Scientific paper

While for type-2 AGN the 9.7 micron absorption from amorphous silicates is well known, the silicate emission feature predicted for type-1 AGN has not been observed so far. Now, our low resolution IRS spectra of two ultra-luminous Palomar-Green quasars show a broad emission feature between 9 and 13 micron rest frame wavelength which contributes about 20% of the quasars total luminosity in this wavelength band. We strongly suggest that this feature is due to silicates in emission. If true, then this result is spectacular in that it provides excellent spectroscopic evidence for the torus-like distribution of dust around quasars, a crucial requirement for the AGN unified schemes. Also remarkable is the fact that the emission feature is not located at rest frame wavelength 9.7 micron typical for amorphous silicates, rather it lies red-shifted in the wavelength range of crystalline silicates. While amorphous silicates are formed in cold regions, crystalline silicates require hot environments. Furthermore, at high spectral resolution ISOSWS observations have shown that, for example in Herbig Ae-Be stars, the crystalline silicates exhibit a family of several sharp lines. At the low resolution of our IRS spectra this family of lines will be smeared out, probably resulting in the single broad feature we see in those two quasars yet observed. Resolving the family of crystalline silicate lines will provide best evidence for the silicate nature of the broad emission bump seen in our quasars. Therefore, we propose to perform high resolution IRS spectroscopy of the two bright quasars 3C249.1 (= PG 1100+772) and 3C351 (= PG 1704+608); at their redshift about z=0.3 the rest-frame 9-13 micron bump shifts to 12-18 micron, hence is ideally covered by the IRS Short-HiRes mode. These observations will establish the nature of the broad emission feature as crystalline silicates, thereby providing unique templates for dust features in luminous AGN.

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