The Nuclear Ten Micron Emission of Spiral Galaxies

We examine the 10\m\ emission of the central regions of 281 spiral galaxies, after having compiled all ground-based, small-aperture ($\sim$5") broad-band photometric observations at \l$\sim$10\m\ (N magnitudes) published in the literature. We evaluate the compactness of the $\sim$10\m\ emission of galaxy nuclei by comparing these small-beam measures with the large-beam IRAS 12\m\ fluxes. In the analysis of different subsets of objects, we apply survival analysis techniques in order to exploit the information contained in ``censored'' data (i.e., upper limits on the fluxes). Seyferts are found to contain the most powerful nuclear sources of mid-infrared emission, which in $\sim$1/3 of cases provide the bulk of the emission of the entire galaxy; thus, mid-infrared emission in the outer disc regions is not uncommon in Seyferts. The 10\m\ emission of Seyferts appears to be unrelated to their X-ray emission. HII region-like nuclei are stronger mid-infrared sources than normal nuclei and LINER nuclei (whose level of emission is not distinguishable from that of normal nuclei). Interacting objects have, on average, greater 10\m\ luminosities than non-interacting ones and exhibit more compact emission. Early-type spirals have stronger and more compact 10\m\ emission than late-type ones. Barred spirals are brighter at $\sim$10\m\ than unbarred systems, essentially because they more frequently contain HII region-like nuclei. The results of our detailed comparison between the behaviour of various categories of objects stress that the 10\m\ emission of spiral nuclei is closely linked to the (predominantly non-thermal synchrotron) radio