Dust in active galactic nuclei

Abstract

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 to study the MIR emission of AGN. We have 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 allows us to probe physical regions of 57 pc (median). We have used a novel pipeline called RedCan capable of producing flux- and wavelength-calibrated spectra for the CanariCam (GTC) and T-ReCS (Gemini) instruments. We have measured the fine-structure [SIV] at 10.5 microns and the PAH at 11.3 microns line strengths together with the silicate absorption/emission features. We have also compiled Spitzer/IRS spectra to understand how spatial resolution influences the results. The 11.3 microns 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 NGC7130 accounts for more than 80% of the MIR continuum at 12 microns while in the case of NGC1808 the AGN is not dominating the MIR emission. This is confirmed by the correlation between the MIR and X-ray continuum luminosities. The [SIV] emission line at 10.5 microns, 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 microns 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.