The Opacity of the Galactic Disk derived with Planetary Nebulae
ABSTRACT Planetary nebulae of the Galactic bulge are used as background sources to probe the extinction in the disk. A systematic decrease of the extinctions with galactic latitude is found as well as a genuine scatter about the mean relation. Both are well accounted for by a model of small clouds randomly distributed in an exponential disk similar to the gas disk, with average cloud extinctions taken from the classical models derived from solar neighbourhood stars. The latter models thus provide also an excellent description for the global extinction of the disk. The pole-to-pole extinction of the Milky Way is found as A_V = 1.4, and in the plane one has A_V = 27 to the centre, in agreement with far-IR studies, but also with individual external galaxies. This indicates that our Galaxy is optically thin, a property shared with other spirals. Observable properties of galactic disks with our extinction model, as would be seen in external galaxies, are presented.