The Compton-Thick Seyfert 2 Nucleus of NGC3281: Torus Constraints from the 9.7$\mu$m Silicate Absorption

Abstract

We present mid infrared (Mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC\,3281, obtained with the Thermal-Region Camera Spectrograph (T-ReCS) at the Gemini South telescope. The spectra present a very deep silicate absorption at 9.7\,$\mu$m, and [S{\sc\,iv]}\,10.5\,$\mu$m and [Ne{\sc\,ii]}\,12.7\,$\mu$m ionic lines, but no evidence of PAH emission. We find that the nuclear optical extinction is in the range 24 $\leq$ A$_{V}$ $\leq$ 83\,mag. A temperature T = 300\,K was found for the black-body dust continuum component of the unresolved 65\,pc nucleus and at 130\,pc SE, while the region at 130\,pc reveals a colder temperature (200\,K). We describe the nuclear spectrum of NGC\,3281 using a clumpy torus model that suggests that the nucleus of this galaxy hosts a dusty toroidal structure. According to this model, the ratio between the inner and outer radius of the torus in NGC\,3281 is $R_0/R_d$ = 20, with {\bf 14} clouds in the equatorial radius with optical depth of $\tau_{V}$ = 40\,mag. We would be looking in the direction of the torus equatorial radius ($i$ = {\bf 60$^{\circ}$}), which has outer radius of R$_{0}\,\sim$ 11\,pc. The column density is N$_{H}\approx$\,{\bf 1.2}\,$\times\,10^{24}\,cm^{-2}$ and iron K$\alpha$ equivalent width ($\approx$ 0.5 - 1.2\,keV) are used to check the torus geometry. Our findings indicate that the X-ray absorbing column density, which classifies NGC\,3281 as a Compton-thick source, may also be responsible for the absorption at 9.7\,$\mu$m providing strong evidence that the silicate dust responsible for this absorption can be located in the AGN torus.