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Register a new userModelling the strongest silicate emission features of local type 1 AGN
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Added by
Mariela Mart\\'inez-Paredes
Publication date
2020
fields
Physics
and research's language is
English
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We measure the 10 and $18mu$m silicate features in a sample of 67 local ($z<0.1$) type 1 active galactic nuclei (AGN) with available {it Spitzer} spectra dominated by non-stellar processes. We find that the $10mu$m silicate feature peaks at $10.3^{+0.7}_{-0.9}mu$m with a strength (Si$_{p}$ = ln f$_{p}$(spectrum)/f$_{p}$(continuum)) of $0.11^{+0.15}_{-0.36}$, while the $18mu$m one peaks at $17.3^{+0.4}_{-0.7}mu$m with a strength of $0.14^{+0.06}_{-0.06}$. We select from this sample sources with the strongest 10$mu$m silicate strength ($sigma_{Si_{10mu m}}>0.28$, 10 objects). We carry out a detailed modeling of the IRS/{it Spitzer} spectra by comparing several models that assume different geometries and dust composition: a smooth torus model, two clumpy torus models, a two-phase medium torus model, and a disk+outflow clumpy model. We find that the silicate features are well modeled by the clumpy model of Nenkova et al. 2008, and among all models those including outflows and complex dust composition are the best (Hoenig et al. 2017). We note that even in AGN-dominated galaxies it is usually necessary to add stellar contributions to reproduce the emission at the shortest wavelengths.
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We report the detection and successful modeling of the unusual 9.7mum Si--O stretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature in the central 230 pc of M81 to be in strong emission, with a peak at ~10.5mum. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and graphite. In addition to silicate dust, there is weak PAH emission present (particularly at 11.3mum, arising from the C--H out-of-plane bending vibration of relatively large PAHs of ~500--1000 C atoms) whose character reflects the low-excitation AGN environment, with some evidence that small PAHs of ~100--200 C atoms (responsible for the 7.7mum C--C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. (abstract continues)
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