High Spatial Resolution Kao Far-infrared Observations of the Central Regions of Infrared-bright Galaxies

We present new high spatial resolution Kuiper Airborne Observatory (KAO) 50 μm and/or 100 μm data for 11 infrared-bright galaxies. The sizes of the central far-infrared emitting regions in three of these galaxies, NGC 660, NGC 891, and M 83, agree with those of the central star formation complexes. The Sb galaxy NGC 7331, which does not have observed star formation in its bulge, has a flat-topped 100 μm major axis profile which covers the bulge and inner spiral arms, without a bright central peak or pronounced depression. The remaining seven galaxies, all of which are known to have strong nuclear or circumnuclear star formation, are unresolved or marginally resolved with the KAO, with farinfrared source size limits consistent with the sizes of the central star formation complexes. Along with these new data, we have tabulated previously published KAO data for 11 other galaxies, and IRAS 60 and 100 μm data for the bulges of the large angular size galaxies M 31 and M 81. From the literature, we have compiled optical, near-infrared, and millimeter measurements for the central regions of the entire set of 24 galaxies. We have used this dataset to investigate dust heating and star formation in the central areas of galaxies. We find that L(FIR)/L(B) and L(FIR)/L(H) correlate with CO (1 − 0) intensity and 100 μm optical depth. Galaxies with optical or near-infrared signatures of OB star formation in their central regions have higher values of I(CO) and τ100 than more quiescent galaxies, as well as higher far-infrared surface brightnesses and L(FIR)/L(B) and L(FIR)/L(H) ratios. The L(FIR)/L(Hα) ratio does not correlate strongly with CO and τ100. These data are consistent with a scenario in which OB stars dominate dust heating in the more active galaxies and older stars are important for the more quiescent bulges. Whether or not a galaxy bulge has strong star formation may be decided by a threshold effect; star forming galaxies have surface 3 gas densities above the Kennicutt critical density, while quiescent galaxies have lower values.