2009A&A...493...23K

We attempt to determine the optical depth of spiral galaxy disks by a statistical study of new Tully-Fisher data from the ongoing KLUN+ survey, and to clarify the difference between the true and apparent behavior of optical depth. By utilizing so-called normalized distances, a subsample of the data is identified to be as free from selection effects as possible. For these galaxies, a set of apparent quantities are calculated for face-on positions using the Tully-Fisher diameter and magnitude relations. These values are compared with direct observations to determine the mean value of the parameter C describing the optical depth. The present study suggests that spiral galaxy disks are relatively optically thin τ_B≃0.1, at least in the outermost regions, while they appear in general to be optically thick τ_B>1 when the apparent magnitude and average surface brightness are studied statistically.