Direct measurement of the optical depth in a spiral galaxy

Abstract

FROM a statistical analysis of nearly 9,400 spiral galaxies¹, Valentijn² has claimed that the disks of spirals are largely opaque. His argument derives from a lack of inclination dependence in the average surface brightness of the spirals, which if they were transparent would be brighter when seen edge-on than face-on. This statistically derived result is however vulnerable to several selection effects³, and seems to contradict the fact that the survey was successfully performed (as we live in a transparent spiral galaxy) as well as anecdotal examples of galaxies visible through other galaxies. We have tried to measure directly optical extinction in a number of spiral disks, using pairs in which a foreground spiral is backlit by another galaxy. In our best example substantial extinction of starlight occurs, but the extinction is mostly associated with the spiral features in which much of the light arises in the first place. This correlation of extinction with emission may account for both the transparency and apparent opacity of spiral galaxies.