FAR-INFRARED EMISSION FROM THE BULGES OF EARLY-TYPE SPIRALS - KAO OBSERVATIONS OF NGC-4736 (M94) AND NGC-3627 (M66)

We present new high spatial resolution Kuiper Airborne Observatory 50 and 100 mum measurements of the ringed Sab galaxy NGC 4736 and the Sb galaxy NGC 3627. We detect strong far-infrared emission (approximately 10(9) L.) from the bulge areas of these galaxies, regions where there is little observed star formation. This far-infrared radiation in ext ended over the bulge, with spatial distributions similar to that of th e old stellar population. The ratio of far-infrared of Halpha luminosi ty for these central sources is significantly larger than for the star -forming regions in these galaxies. This ratio is higher than can be a ccounted for by dust heating due to a population of young stars with a mass distribution similar to the Salpeter initial mass function, assu ming current estimates of extinction, unless the upper mass limit is u nusually low (approximately B1 stars). On the other hand, the bolometr ic luminosity of the observed bulge stars is sufficient to account for the far-infrared radiation. Thus, older stars are likely to be import ant in heating dust in these bulges. The dust in the centers of these galaxies is quite warm, approximately 40 K (lambda-1 emissivity), howe ver, the interstellar radiation fields derived from the optical lumino sities are consistent with these temperatures without invoking hidden central star formation or a dusty active galactic nucleus.