Revisiting the infrared spectra of active galactic nuclei with a new torus emission model

Abstract

We describe improved modelling of the emission by dust in a toroidal-like structure heated by a central illuminating source within active galactic nuclei (AGNs). We have chosen a simple but realistic torus geometry, a flared disc, and a dust grain distribution function including a full range of grain sizes. The optical depth within the torus is computed in detail taking into account the different sublimation temperatures of the silicate and graphite grains, which solves previously reported inconsistencies in the silicate emission feature in type 1 AGNs. We exploit this model to study the spectral energy distributions (SEDs) of 58 extragalactic (both type 1 and type 2) sources using archival optical and infrared data. We find that both AGN and starburst contributions are often required to reproduce the observed SEDs, although in a few cases they are very well fitted by a pure AGN component. The AGN contribution to the far-infrared luminosity is found to be higher in type 1 sources, with all the type 2 requiring a substantial contribution from a circumnuclear starburst. Our results appear in agreement with the AGN unified scheme, because the distributions of key parameters of the torus models turn out to be compatible for type 1 and type 2 AGNs. Further support to the unification concept comes from comparison with medium-resolution infrared spectra of type 1 AGNs by the Spitzer observatory, showing evidence for a moderate silicate emission around 10 μm, which our code reproduces. From our analysis we infer accretion flows in the inner nucleus of local AGNs characterized by high equatorial optical depths (A V 100), moderate sizes (R max < 100 pc) and very high covering factors ( f 80 per cent) on average.

Dusty star-forming galaxies at high redshift

“…When discussing these models, it's important to keep in mind that the accuracy or applicability of these models cannot be tested with data since it simply does not exist in enough detail to disentangle effects of geometry, distribution, optical depth, etc. It's also important to note that many have done work in this area, particularly modeling radiative transfer in local starburst populations to generate SEDs (Efstathiou & Rowan-Robinson, 1995;Efstathiou et al, 2000;Efstathiou & Siebenmorgen, 2009;Nenkova et al, 2002;Dullemond & van Bemmel, 2005;Piovan et al, 2006;Nenkova et al, 2008;Takagi et al, 2003;Fritz et al, 2006;Hönig et al, 2006;Schartmann et al, 2008), but here we try to focus on the techniques which have been most commonly employed for SED fitting of high-z dusty starbursts 11 . Silva et al (1998) developed the Grasil code to model galaxy emission by explicitly accounting for dust absorption and emission from the ultraviolet through to the far-infrared.…”

The near-to-mid infrared spectrum of quasars

“…A power-law extension of the restframe optical disk emission into the NIR is a common assumption for the primary source that is used as input for radiative transfer codes that model the torus emission (e.g. Granato & Danese 1994;Nenkova et al 2002;Fritz et al 2006;Stalevski et al 2012), but it is not universal. In particular, some torus models assume a steep fall of the disk spectrum longwards of 1 µm (e.g.…”

The XMM deep survey in the CDF-S

“…The spectral energy distribution (SED) of PID352 was modeled using the SED-fitting code originally developed by Fritz et al (2006) and recently updated by Feltre et al (2012), and assuming z = 1.60 as derived from our X-ray spectral analysis. We note that, within the errors, the X-ray-derived redshift is consistent with both current photometric-redshift estimates (Taylor et al 2009;Hsu et al 2014), and it is currently the one with the lowest uncertainties.…”