THE EDGE OF THE GALACTIC DISK

As part of a stellar population sampling program, a series of photometric probes at various field sizes and depths have been obtained in a low-extinction window in the Galactic anticenter direction. Such data set strong constraints on the radial structure of the disk. At the forefront of this "drilling" program, very deep CCD frames probe the most external parts of the disk. Over the whole effective magnitude range (18-25), all contributions in the statistics which should be expected from old disk stars beyond 6 kpc vanish, although such stars dominate by far at distances less than 5 kpc. This is the signature of a sharp cutoff in the star density: the edge of the Galactic disk between 5.5 and 6 kpc. As a consequence, the Galactic radius does not exceed 14 kpc (assuming R. = 8.5). Colors of elliptical galaxies measured in the field rule out the risk of being misled by undetected extinction.