VLT diffraction-limited imaging at 11 and 18 μm of the nearest active galactic nuclei

Abstract

Mid-infrared (mid-IR) imaging at resolutions of 300 mas of the central kpc region of 13 nearby, well-known active galaxies is presented. The bulk of the mid-IR emission is concentrated on an unresolved central source within a size of less than 5–130 pc, depending on the object distance. Further resolved emission is detected in 70 per cent of the sample in the form of circumnuclear star-forming rings or diffuse nuclear extended emission. In the three cases with circumnuclear star formation, the stellar contribution is at least as important as that of the active galactic nuclei (AGN). In those with extended nuclear emission – a third of the sample – this emission represents a few per cent of the total measured; however, this contribution may be underestimated because of the chopped nature of these observations. This extended emission is generally collimated in a preferential direction often coinciding with that of the extended ionized gas or the jet. In M87 and Cen A, where the emission extends along their respective jets, the emission is presumably synchrotron. In Circinus, NGC 1386 and NGC 3783, it can be reconciled with thermal emission from dust heated at about 100 K by the active nucleus. In all cases, the nuclear fluxes measured at 11.8 and 18.7 μm represent a minor contribution of the flux levels measured by large aperture Infrared Astronomical Satellite (IRAS) data at the nearest energy bands of 12 and 25 μm. This contribution ranges from 30 per cent to less than 10 per cent. In only three cases do the AGN fluxes agree with IRAS to within a factor of 2. In the AGN with strong circumnuclear star formation, this component can well account for most of the IRAS flux measured in these objects. But in all other cases, either a low surface brightness component extending over galactic scales or strong extra-nuclear IR sources – e.g. H ii regions in spiral arms – have to be the main source of the IRAS emission. In either case, the contribution of these components dwarfs that of the AGN at mid-IR wavelengths.