THE OPACITY OF THE GALACTIC DISC DERIVED WITH PLANETARY-NEBULAE

Planetary nebulae of the Galactic bulge are used as background sources to probe the extinction in the disc. A systematic decrease of the ext inctions with galactic latitude is found, as well as a genuine scatter about the mean relation. Both are well accounted for by a model of sm all clouds randomly distributed in an exponential disc similar to the gas disc, with average cloud extinctions taken from the classical mode ls derived from solar neighbourhood stars. The latter models thus also provide an excellent description for the global extinction of the dis c. The pole-to-pole extinction of the Milky Way is found to be A(v) = 1.4, and in the plane one has A(v) = 27 to the centre, in agreement bo th with far-LR studies and with individual external galaxies. This ind icates that our Galaxy is optically thin, a property shared with other spirals. Observable properties of galactic discs with our extinction model, as would be seen in external galaxies, are presented.