SCALE LENGTHS IN DISK SURFACE BRIGHTNESS AS PROBES OF DUST EXTINCTIONIN 3 SPIRAL GALAXIES - M51, NGC-3631, AND NGC-4321

We have measured the radial brightness distributions in the disks of t hree nearby face-on spiral galaxies, M51, NGC 3631, and NGC 4321 (M100 ), in the photometric bands B through I, with the addition of the K ba nd for M51 only. The measurements were made by averaging azimuthally, in three modes, the two-dimensional surface brightness over the disks in photometric images of the objects in each band: (1) over each disk as a whole, (2) over the spiral arms alone, and (3) over the interarm zones alone. From these profiles, scale lengths were derived for compa rison with schematic exponential disk models that incorporate interste llar dust. These models include both absorption and scattering in thei r treatment of radiative transfer. The model fits show that the arms e xhibit greater optical depth in dust than the interarm zones. The aver age fraction of emitted stellar light in V that is extinguished by dus t within 3 scale lengths of the center of each galaxy does not rise ab ove 20% in any of them. We show that this conclusion is also valid for models with similar overall quantities of dust but in which this is c oncentrated in lanes. These can also account for the observed scale le ngths and their variations.