The distribution of galactic inclinations - A clue to opacity?

Although it has proved possible to determine the optical depth along individual lines of sight to specific objects within or behind galaxies the issue of the mean optical depth over a 'typical' galactic disc remains open. The classical statistical test of opacity is to consider the change in surface brightness with inclination. We have shown how selection effects have brought into question the interpretation of previous such tests which have indicated high opacity. We have also shown how realistic star dust geometries should always lead to a surface-brightness inclination test result that indicates low opacity irrespective of the true opacity. In this paper we consider an alternative statistical approach: the distribution of galactic inclinations. We compare the observed distribution of galactic inclinations (as measured in the region of the 25 B mu isophote) taken from the UGC, RC3 and ESO catalogues,with simulated samples. We conclude that the observational data are best fitted by an optically thin model, thus confirming our previous assertion that all such tests should indicate low opacities. This is contrary to the results of recently published surface-brightness inclination tests.