EXTINCTION IN SC GALAXIES

We analyze the photometric properties of a sample of Sbc-Sc galaxies with 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 find spiral galaxies to be substantially less transparent than suggested in most previous determinations, but not as opaque as claimed by Valentijn [Nature, 346, 153 (1990)]. Regions in the disk farther than two or three scale lengths from the center are close to completely transparent. In addition to statistically derived relations for the inclination dependence of photometric parameters, we present the results of a modeling exercise that utilizes the ''triplex'' model of Disney et al. [MNRAS, 239, 939 (1989)] to obtain upper limits of the disk opacity. Within the framework of that model, and with qualitative consideration of the effects of scattering on extinction, we estimate late spiral disks at I band to have central optical depths tau(I)(0)<5 and dust absorbing layers with scale heights on the order of half that of the stellar component or less. We discuss our results in light of previous determinations of internal extinction relations and point out the substantial impact of internal extinction on the scatter of the Tully-Fisher relation. We also find that the visual diameters by which large catalogs are constructed (UGC, ESO-Uppsala) are nearly proportional to face-on isophotal diameters.