THE NUCLEAR 10 MICRON EMISSION OF SPIRAL GALAXIES

We examine the 10 mum emission of the central regions of 281 spiral galaxies, after having compiled all ground-based, small-aperture (approximately 5'') broad-band photometric observations at lambda approximately 10 mum (N magnitudes) published in the literature. We evaluate the compactness of the approximately 10 mum emission of galaxy nuclei by comparing these small-beam measures with the large-beam IRAS 12 mum 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). Seyfert galaxies are found to contain the most powerful nuclear sources of mid-infrared emission, which in approximately one-third of the cases provide the bulk of the emission of the entire galaxy; thus, mid-infrared emission in the outer disk regions is not uncommon in Seyfert galaxies, The 10 mum emission of Seyfert galaxies appears to be unrelated to their X-ray emission. H II 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 mum luminosities than noninteracting ones and exhibit more compact emission. Early-type spirals have stronger and more compact 10 mum emission than late-type ones. Barred spirals are brighter at approximately 10 mum than unbarred systems, essentially because they more frequently contain H II region-like nuclei. The results of our detailed comparison between the behavior of various categories of objects stress that the 10 mum emission of spiral nuclei is closely linked to the (predominantly nonthermal synchrotron) radio emission.