Testing the Unification Model for Active Galactic Nuclei in the Infrared: Are the Obscuring Tori of Type 1 and 2 Seyferts Different?

Abstract

We present new mid-infrared (MIR) imaging data for three Type-1 Seyfert galaxies obtained with T-ReCS on the Gemini-South Telescope at subarcsecond resolution. Our aim is to enlarge the sample studied in a previous work to compare the properties of Type-1 and Type-2 Seyfert tori using clumpy torus models and a Bayesian approach to fit the infrared nuclear spectral energy distributions (SEDs). Thus, the sample considered here comprises 7 Type-1, 11 Type-2, and 3 intermediate-type Seyferts. The unresolved IR emission of the Seyfert 1 galaxies can be reproduced by a combination of dust heated by the central engine and direct AGN emission, while for the Seyfert 2 nuclei only dust emission is considered. These dusty tori have physical sizes smaller than 6 pc radius, as derived from our fits. Unification schemes of AGN account for a variety of observational differences in terms of viewing geometry. However, we find evidence that strong unification may not hold, and that the immediate dusty surroundings of Type-1 and Type-2 Seyfert nuclei are intrinsically different. The Type-2 tori studied here are broader, have more clumps, and these clumps have lower optical depths than those of Type-1 tori. The larger the covering factor of the torus, the smaller the probability of having direct view of the AGN, and vice-versa. In our sample, Seyfert 2 tori have larger covering factors (C T =0.95±0.02) and smaller escape probabilities (P esc =0.05± 0.08 0.03 %) than those of Seyfert 1 (C T =0.5±0.1; P esc =18±3 %). All the previous differences are significant according to the Kullback-Leibler divergence. Thus, on the basis of the results presented here, the classification of a Seyfert galaxy as a Type-1 or Type-2 depends more on the intrinsic properties of the torus rather than on its mere inclination towards us, in contradiction with the simplest unification model.

Nuclear obscuration in LINERs

“…9, together with 68% confidence intervals for all the parameters (see Ramos Almeida et al 2009, for more details on the modelling). As already shown by Ramos Almeida et al (2011), the continuum shape of S1s and S2s are very represented well by clumpy models (panels a and b). Bright LINERs (i.e.…”

“…Errors were estimated assuming 15% flux-calibration uncertainties, which fully dominate other source of errors (e.g. González-Martín et al 2013;Ramos Almeida et al 2011). We also measured the fluxes and equivalent width (EW) of the polycyclic aromatic hydrocarbon (PAH) features at 6.2 and 11.3 µm.…”

“…We refer here to the steepness at the spatial resolutions of the Spitzer/IRS spectra (i.e. kpc scales), while on smaller scales (below 100 pc scales obtained with ground-based instruments), the mid-IR emission is expected to be dominated from dust heated by the AGN Ramos Almeida et al 2011). It has proven to be a good indicator of the SB content given its correlation with EW(PAH 11.3 µm) (e.g.…”

Synapses of active galactic nuclei:

“…The torus must not be spherically symmetric to obscure the BLR, so that at the same time the region producing the permitted narrow lines (known as narrow-line region, NLR) reaches us from the same LOS. The locus of this obscuring material was initially postulated at parsec scales and confirmed by modelling the spectral energy distribution (SED) of Seyferts (e.g., Ramos Almeida et al 2011;Alonso-Herrero et al 2011) and by interferometric observations (e.g., Circinus galaxy, Tristram et al 2007). Such scales are beyond the current instrumentation, therefore the torus morphology can only be inferred by indirect measurements.…”

X-ray long-term variations in the low-luminosity AGN NGC 835 and its circumnuclear emission

“…Thus, mid-infrared observations can give good evidence of the dust heated by the AGN in the very centre (e.g. Ramos Almeida et al 2011), together with information on the star formation when present (e.g. Esquej et al 2014;Alonso-Herrero et al 2014).…”