SURFACE PHOTOMETRY OF DISK GALAXIES AT 60 AND 100 MU-M - RADIAL GRADIENTS IN DUST TEMPERATURE IN OPTICALLY THIN DISKS

Surface, aperture, and grid photometry are performed at 60 and 100-mu-m for a sample of 28 nearby, large angular size galaxies. The spatial resolution of the data is 0.5-2.0 kpc, which allows for a coarse investigation of predicted radial gradients in physical dust temperature. We derive a model which predicts a drop of 3 K for every optical scale length, and this model agrees well with the data. In addition, for a subsample of 16 galaxies we directly compare the radial profile at 60-mu-m with that in the B or I band. Our model predicts that optically thick disks should have scale lengths a factor of 2 smaller at 60-mu-m compared to B. In the optically thin case, the scale lengths are about equal except for the effect of the gradient in dust temperature. Virtually all of the data support the prediction of the optically thin case. Finally, 10 galaxies in the sample have sufficient angular sizes to allow for a coarse 2D spatial mapping of the 60/100-mu-m flux ratio. These maps clearly reveal that, when present, small amounts of hot dust dominate the emission at 60-mu-m and effectively mask the existence of a much larger amount of cold dust.