On the 10-micron silicate feature in Active Galactic Nuclei

The 10-micron silicate feature observed with Spitzer in active galactic nuclei (AGN) 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. (1) We present the spectral energy distribution (SED) of SST1721+6012, the first type 2 quasar observed to show a clear 10-mic silicate feature in emission. We constructed a large database of clumpy torus models and performed extensive fitting of the observed SED, constraining several of the torus parameters. We find that the source bolometric luminosity is ~3*10^12 L_sun. Our modeling suggests that <35% of objects with tori sharing characteristics and geometry similar to the best fit would have their central engines obscured. This relatively low obscuration probability can explain the clear appearance of the 10-mic emission feature in SST1721+6012 together with its rarity among other QSO2. (2) We also fitted the SED of PG1211+143, one of the first type 1 QSOs with a 10-mic silicate feature in emission. Among 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 radiative transfer effects. (3) We find that the distribution of silicate feature strengths among clumpy torus models closely resembles the observed distribution, and the feature never occurs deeply absorbed. (abridged)Comment: 11 pages, 9 figures, 4 tables, accepted for publication in ApJ; minor revision, added reference in Section

The Multi-wavelength Extreme Starburst Sample of Luminous Galaxies Part I: Sample Characteristics

This paper introduces the Multi-wavelength Extreme Starburst Sample (MESS), a new catalog of 138 star-forming galaxies (0.1 < z < 0.3) optically selected from the SDSS using emission line strength diagnostics to have high absolute SFR (minimum 11 solar masses per year, with median SFR approx 61 solar masses per year based on a Kroupa IMF). The MESS was designed to complement samples of nearby star-forming galaxies such as the luminous infrared galaxies (LIRGs), and ultraviolet luminous galaxies (UVLGs). Observations using the multiband imaging photometer (MIPS; 24, 70, and 160{\mu}m channels) on the Spitzer Space Telescope indicate the MESS galaxies have IR luminosities similar to those of LIRGs, with an estimated median LTIR ~ 3e11 solar luminosities. The selection criteria for the MESS suggests they may be less obscured than typical far-IR selected galaxies with similar estimated SFRs. 20 out of 70 of the MESS objects detected in the GALEX FUV band also appear to be UV luminous galaxies. We estimate the SFRs based directly on luminosities to determine the agreement for these methods in the MESS. We compare to the emission line strength technique, since effective measurement of dust attenuation plays a central role in these methods. We apply an image stacking technique to the VLA FIRST survey radio data to retrieve 1.4 GHz luminosity information for 3/4 of the sample covered by FIRST including sources too faint, and at too high a redshift, to be detected in FIRST. We also discuss the relationship between the MESS and samples selected through alternative criteria. Morphologies will be the subject of a forthcoming paper.Comment: 59 pages, 19 figures, accepted for publication in A