The radial structure of the Galactic disc

Abstract

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 anticentre direction. Such data set strong constraints on the radial structure of the disc. These new data, used in combination with lower magnitude photographic data in a wider field, give a strong evidence that the galactic density scale length is rather short (2.5 kpc) and drops abruptly beyond 6 kpc. Over the whole effective magnitude range (12 to 25), all contributions in the statistics which should be expected from old disc stars beyond 6 kpc vanish, although such stars dominate by far at distances less than 5 kpc. This is the signature of a sharp cut‐off in the star density: the edge of the galactic disc between 5.5 and 6 kpc. As a consequence, the galactic radius does not exceed 14 kpc (assuming Ro=8.5). Colors of elliptical galaxies measured in the field rule out the risk of being misled by undetected extinction.