EXTINCTION IN SC GALAXIES

We analyze the photometric properties of a sample of Sbc-Sc galaxies w ith known redshifts, single-dish H I profiles, and CCD I band images. We derive laws that relate the measured isophotal radius at mu(I) = 23 .5, magnitude, scale length, and H I flux to the face-on aspect. We fi nd spiral galaxies to be substantially less transparent than suggested in most previous determinations, but not as opaque as claimed by Vale ntijn [Nature, 346, 153 (1990)]. Regions in the disk farther than two or three scale lengths from the center are close to completely transpa rent. In addition to statistically derived relations for the inclinati on dependence of photometric parameters, we present the results of a m odeling exercise that utilizes the ''triplex'' model of Disney et al. [MNRAS, 239, 939 (1989)] to obtain upper limits of the disk opacity. W ithin the framework of that model, and with qualitative consideration of the effects of scattering on extinction, we estimate late spiral di sks at I band to have central optical depths tau(I)(0)<5 and dust abso rbing layers with scale heights on the order of half that of the stell ar component or less. We discuss our results in light of previous dete rminations of internal extinction relations and point out the substant ial impact of internal extinction on the scatter of the Tully-Fisher r elation. We also find that the visual diameters by which large catalog s are constructed (UGC, ESO-Uppsala) are nearly proportional to face-o n isophotal diameters.