THE INFRARED BRIGHT NUCLEI IN THE MIDINFRARED

We present arcsecond angular-resolution mid-IR images of three IR-brig ht galaxies, NGC 6240, IRAS 0518-25, and the NGC 3690-IC 694 (Arp 299) system, and we compare the sizes of their emission regions in the mid -IR and near-IR, In all these galaxies, most (similar to 75%) of the m id-IR flux, as measured in the IRAS 12 mu m observations, is found in our images to be concentrated within the central few hundred parsecs. Hn the two galaxies which shaw a single IB source in the nucleus, IRAS 0518-25 and IC 694, the mid-IR emission is more compact than the near -IR. This suggests that the mid-IR source, warm dust heated by an acti ve galactic nucleus or an unresolved starburst, is more compact than t he population of cooler, older stars. In the two galaxies which show d ouble sources in the nucleus, the reverse is the case, That is, the mi d-IR is more extended than the near-IR. Similar results are obtained f or a handful of other galaxies for which we have high-resolution mid-I R data, Thus in our small sample, the double source nuclei contain sta rbursts which are more extended than the older stars and the single so urce nuclei do not. Some of the double sources seen in the IR bright g alaxies, for example NGC 3690, are probably starbursts in conjunction with a galactic nucleus rather than twin galactic nuclei.