Nuclear activity in nearby galaxies - Mid-infrared imaging with the VLT

Dust enshrouded activity can be studied ideally by mid-infrared (MIR) observations. To explore the active galactic nuclei (AGN) versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. We report on 11.3 mu m observations with VISIR at the VLT, which reach 0.35 '' spatial resolution (FWHM). Methods. During the scientific verification of VISIR, we observed a sample of 36 nearby galaxies with a range of optically classified nuclear activity: 17 black hole driven AGN, 10 starbursts (SB), and 9 quiescent spirals. Results. 16 objects out of 17 AGN are detected and unresolved, 5 out of 10 SB are detected and resolved with structured emission up to a few arcsec, while for 5 targets out of 10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei is similar to that observed at radio frequencies. The compactness of AGN and the extent of SB nuclei is consistent with the predictions of radiative transfer models and Spitzer spectra observed at much lower spatial resolution than with VISIR. We introduce the nuclear MIR surface brightness, S, as a quantitative measurement for AGN and SB activity. Although one is unable to distinguish between AGN and SB activity using the nuclear MIR surface brightness derived from 4 m class telescopes, our data indicate that, out to a distance of 100 Mpc, the MIR surface brightness acquired by 8 m class telescopes can differentiate AGN from SB behavior.