The nuclear and extended mid-infrared emission of Seyfert galaxies

We present subarcsecond resolution mid-infrared (MIR) images obtained with 8-10 m-class ground-based telescopes of a complete volume-limited (D-L < 40 Mpc) sample of 24 Seyfert galaxies selected from the Swift/Burst Alert Telescope nine month catalogue. We use those MIR images to study the nuclear and circumnuclear emission of the galaxies. Using different methods to classify the MIR morphologies on scales of similar to 400 pc, we find that the majority of the galaxies (75-83 per cent) are extended or possibly extended and 17-25 per cent are point-like. This extended emission is compact and it has low surface brightness compared with the nuclear emission, and it represents, on average, similar to 30 per cent of the total MIR emission of the galaxies in the sample. We find that the galaxies whose circumnuclear MIR emission is dominated by star formation (SF) show more extended emission (650 +/- 700 pc) than active galactic nuclei (AGN)-dominated systems (300 +/- 100 pc). In general, the galaxies with point-like MIR morphologies are face-on or moderately inclined (b/a similar to 0.4-1.0), and we do not find significant differences between the morphologies of Sy1 and Sy2. We used the nuclear and circumnuclear fluxes to investigate their correlation with different AGN and SF activity indicators. We find that the nuclear MIR emission (the inner similar to 70 pc) is strongly correlated with the X-ray emission (the harder the X-rays the better the correlation) and with the [O IV] lambda 25.89 mu m emission line, indicating that it is AGN-dominated. We find the same results, although with more scatter, for the circumnuclear emission, which indicates that the AGN dominates the MIR emission in the inner similar to 400 pc of the galaxies, with some contribution from SF.