TESTS FOR DUST OPACITY OF SPIRAL GALAXIES

The dust opacity of spiral galaxies can be examined by direct methods, which are free from statistical biases and selection effects. For exa ctly edge-on galaxies, the distribution of optical rotation velocity a long the midplane of the disk reflects the degree of opacity. For incl ined galaxies, dust obscuration can be measured from the asymmetric lu minosity distribution in the direction parallel to the minor axis. We applied these two opacity tests to a sample of galaxies. The results s uggest that spiral disks are not optically thick as claimed by other r ecent studies.