ARE SPIRAL DISKS REALLY OPAQUE

We compare the ultra-violet, optical, and far-infrared emission for a sample of 135 spiral galaxies in order to address the widely debated p roblem concerning the opacity of spiral disks. We find that the re-rad iation of the dust, estimated from the far-infrared emission, is on av erage only 31+/-1 percent of the bolometric luminosity of a spiral gal axy, indicating that less than one third of the stellar radiation is a bsorbed and then re-radiated by dust in a spiral disk. Applying a radi ation transfer model which assumes a 'Sandwich' configuration for the spiral disk, and fully takes into account the effect of scattering, we find for our sample a median of the face-on blue (4400 Angstrom) opti cal depth tau(B) = 0.49 and the mean [tau(B)]= 0.60+/-0.04, indicating that most spiral galaxies in our sample are not opaque for blue light (tau(B) < 1).