Dust in active galactic nuclei

Context. The unified model of active galactic nuclei (AGN) claims that the properties of AGN depend on the viewing angle of the observer with respect to a toroidal distribution of dust surrounding the nucleus. Both the mid-infrared (MIR) attenuation and continuum luminosity are expected to be related to dust associated with the torus. Therefore, isolating the nuclear component is essential for studying the MIR emission of AGN. Aims. This work is aimed at studying the MIR emission of AGN with the highest spatial resolution available to date, isolating its contribution from extended emission. We would like to address three fundamental questions: (1) how important is the AGN contribution to the MIR spectrum; (2) where dust attenuation arises; and (3) how does spatial resolution affect these issues. Methods. We compiled all the T-ReCS spectra (Gemini Observatory) available in the N-band for 22 AGN: 5 Type-1 and 17 Type-2 AGN. The high angular resolution of the T-ReCS spectra allowed us to probe physical regions of 57 pc (median). We used a novel pipeline called RedCan capable of producing fluxand wavelength-calibrated spectra for the CanariCam (GTC) and T-ReCS (Gemini) instruments. We measured the fine-structure [S IV] at 10.5 μm and the PAH at 11.3 μm line strengths, together with the silicate absorption/emission features. We also compiled Spitzer/IRS spectra to understand how spatial resolution influences the results. We complemented our sample with the results of 19 VISIR/VLT spectra (Paranal Observatory) and 20 nearby, highly obscured AGN (NH > 1.5 × 1024 cm−2) Spitzer spectra. Results. The 11.3 μm PAH feature is only clearly detected in the nuclear spectra of two AGN, while it is more common in the Spitzer data. For those two objects, the AGN emission in NGC 7130 accounts for more than 80% of the MIR continuum at 12 μm, while in the case of NGC 1808 the AGN does not dominate the MIR emission. This is confirmed by the correlation between the MIR and X-ray continuum luminosities. The [S IV] emission line at 10.5 μm, which is believed to originate in the narrow line region, is detected in most AGN. We have found an enhancement of the optical depth at 9.7 μm (τ9.7) in the high-angular resolution data for higher values of NH. Clumpy torus models reproduce the observed values only if the host-galaxy properties are taken into account.