Mid-infrared imaging of 25 local AGN with VLT-VISIR

TL;DR

High-resolution mid-infrared imaging of 25 local AGN reveals that most nuclei are point-like with some extended emission, supporting the unified AGN model and indicating contamination in lower-resolution spectra.

Contribution

This study provides high-angular resolution VLT-VISIR images and N-band spectra of low-redshift AGN, highlighting the impact of extended emission on mid-infrared observations and supporting clumpy torus models.

Findings

Most AGN nuclei are point-like at 0.35" resolution.

Extended circumnuclear emission detected in some sources.

Spitzer spectra often contaminated by extended emission.

Abstract

Aims. High angular resolution N-band imaging is used to discern the torus of active galactic nuclei (AGN) from its environment in order to allow a comparison of its mid-infrared properties to the expectations of the unified scenario for AGN. Methods. We present VLT-VISIR images of 25 low-redshift AGN of different Seyfert types, as well as N-band SEDs of 20 of them. In addition, we compare our results for 19 of them to Spitzer IRS spectra. Results. We find that at a resolution of ~ 0.35", all the nuclei of our observed sources are point-like, except for 2 objects whose extension is likely of instrumental origin. For 3 objects, however, we observed additional extended circumnuclear emission, even though our observational strategy was not designed to detect it. Comparison of the VISIR photometry and Spitzer spectrophotometry indicates that the latter is affected by extended emission in at least 7 out of 19 objects and the level of contamination is (0.20 ~ 0.85) * F_IRS. In particular, the 10 um silicate emission feature seen in the Spitzer spectra of 6 type I AGN, possibly 1 type II AGN and 2 LINERs, also probably originates not solely in the torus but also in extended regions. Conclusions. Our results generally agree with the expectations from the unified scenario, while the relative weakness of the silicate feature supports clumpy torus models. Our VISIR data indicate that, for low-redshift AGN, a large fraction of Spitzer IRS spectra are contaminated by extended emission close to the AGN.