ON THE 10 mum SILICATE FEATURE IN ACTIVE GALACTIC NUCLEI
- Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055 (United States)
- Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125 (United States)
The 10 mum silicate feature observed with Spitzer in active galactic nuclei (AGNs) reveals some puzzling behavior. It (1) has been detected in emission in type 2 sources, (2) shows broad, flat-topped emission peaks shifted toward long wavelengths in several type 1 sources, and (3) is not seen in deep absorption in any source observed so far. We solve all three puzzles with our clumpy dust radiative transfer formalism. Addressing (1), we present the spectral energy distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a clear 10 mum silicate feature in emission. Such emission arises in models of the AGN torus easily when its clumpy nature is taken into account. We constructed a large database of clumpy torus models and performed extensive fitting of the observed SED. We find that the cloud radial distribution varies as r {sup -1.5} and the torus contains 2-4 clouds along radial equatorial rays, each with optical depth at visual approx60-80. The source bolometric luminosity is approx3 x 10{sup 12} L{sub sun}. Our modeling suggests that approx<35% of objects with tori sharing these characteristics and geometry would have their central engines obscured. This relatively low obscuration probability can explain the clear appearance of the 10 mum emission feature in SST1721+6012 together with its rarity among other QSO2. Investigating (2), we also fitted the SED of PG1211+143, one of the first type 1 QSOs with a 10 mum silicate feature detected in emission. Together with other similar sources, this QSO appears to display an unusually broadened feature whose peak is shifted toward longer wavelengths. Although this led to suggestions of non-standard dust chemistry in these sources, our analysis fits such SEDs with standard galactic dust; the apparent peak shifts arise from simple radiative transfer effects. Regarding (3), we find additionally that the distribution of silicate feature strengths among clumpy torus models closely resembles the observed distribution, and the feature never occurs deeply absorbed. Comparing such distributions in several AGN samples we also show that the silicate emission feature becomes stronger in the transition from Seyfert to quasar luminosities.
- OSTI ID:
- 21392517
- Journal Information:
- Astrophysical Journal, Vol. 707, Issue 2; Other Information: DOI: 10.1088/0004-637X/707/2/1550; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
ABSORPTION
BOLOMETERS
DUSTS
EMISSION
ENERGY SPECTRA
GALAXIES
GALAXY NUCLEI
LUMINOSITY
PROBABILITY
QUASARS
RADIANT HEAT TRANSFER
SILICATES
SIMULATION
SPATIAL DISTRIBUTION
COSMIC RADIO SOURCES
DISTRIBUTION
ENERGY TRANSFER
HEAT TRANSFER
MEASURING INSTRUMENTS
OPTICAL PROPERTIES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SILICON COMPOUNDS
SORPTION
SPECTRA