EXTINCTION IN SC GALAXIES - AN ANALYSIS OF THE ESO-LV DATA

We have analysed the surface photometry of Sc galaxies in the ESO-LV database in view of recent claims by Valentijn that spiral galaxies are opaque over their entire discs. Using diameter-limited and magnitude-limited subsamples, we can discriminate between the opaque and transparent cases with the axis-ratio distribution test. Our results support the traditional view that Sc galaxies are semitransparent, but the brightness difference between face-on and edge-on can be as large as 1.0 to 1.5 mag. Analysing surface-brightness profiles at various radii, we find that the central regions are essentially opaque, and the outer regions nearly fully transparent. We show that our conclusions differ from those of Valentijn, because he neglected to take secondary inclination effects into account.