The compton-thick Seyfert 2 nucleus of NGC 3281 torus constraints from the 9.7μm 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 at the Gemini-South telescope. The spectra present a very deep silicate absorption at 9.7μm, and [S IV] 10.5μm and [Ne II] 12.7μm ionic lines, but no evidence of polycyclic aromatic hydrocarbon emission. We find that the nuclear optical extinction is in the range 24 mag ≤ AV ≤ 83 mag. A temperature T = 300 K was found for the blackbody dust continuum component of the unresolved 65 pc nucleus and the region at 130 pc SE, while the region at 130 pc NW 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 R0/Rd = 20, with 14 clouds in the equatorial radius with optical depth of τV = 40 mag. We would be looking in the direction of the torus equatorial radius (i = 60º), which has outer radius of R0 ∼ 11 pc. The column density is NH ≈ 1.2 × 10 24 cm-² and the iron Kα equivalent width (≈0.5–1.2 keV) is 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 9.7μm providing strong evidence that the silicate dust responsible for this absorption can be located in the active galactic nucleus torus.