The subarcsecond mid-infrared view of local active galactic nuclei: I. The N- and Q-band imaging atlas

We present the first subarcsecond-resolution mid-infrared (MIR) atlas of local active galactic nuclei (AGN) containing 253 objects with a median redshift of z = 0.016. It comprises all available MIR imaging observations performed to date with ground-based 8-meter class telescopes and includes in total 895 independent photometric measurements, of which more than 60% are previously unpublished.We detect extended nuclear emission in at least 21% of the objects, while another 19% appear clearly point-like, and the remaining objects cannot be constrained. Subarcsecond resolution allows us to isolate the emission of the AGN on scales of a few tens of parsecs for the bulk of the sample and obtain nuclear photometry in multiple filters for the objects. The photometry is used to construct median spectral energy distributions (SEDs) for the different optical AGN types and estimate the individual MIR 12 and 18 um continuum luminosities, which range over more than six orders of magnitude. We also analyse the arcsecond-scale MIR emission as probed by Spitzer and compare it to the subarcsecond-scale emission. The continuum emission is on average 20% lower on subarcsecond scales with a large scatter that depends mostly on the object distance or luminosity. The silicate feature strength is similar on both scales and generally appears in emission (absorption) in type I (II) AGN. However, the PAH emission appears weaker or absent on subarcsecond scales, indicating that most of the star formation has been resolved out. The differences of the MIR SEDs on both scales are particularly large for AGN/starburst composites and close-by (and weak) AGN. Because of its size and characteristics, the AGN MIR atlas is well-suited not only for detailed investigation of individual sources but also for sample studies of AGN unification.

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Date added: Fri, 11 Oct 13