The structure of the outer Galactic disc as revealed by IPHAS early A stars

Abstract

This study is an investigation of the stellar density profile of the Galactic disc in the anticentre direction. We select over 40 000 early A stars from INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) photometry in the Galactic longitude range 160° < l < 200° close to the equatorial plane (−1° < b < +1°). We then compare their observed reddening-corrected apparent magnitude distribution with simulated photometry obtained from parametrized models in order to set constraints on the anticentre stellar density profile. By selecting A stars, we are appraising the properties of a population only ∼100 Myr old. We find that the stellar density profile of young stars is well fit to an exponential with length-scale of (3020 ± 120statistical± 180systematic) pc, which is comparable to that obtained in earlier studies, out to a Galactocentric radius of RT= (13.0 ± 0.5statistical± 0.6systematic) kpc. At larger radii, the rate of decline appears to increase with the scalelength dropping to (1200 ± 300statistical± 70systematic) pc. This result amounts to a refinement of the conclusions reached in previous studies that the stellar density profile is abruptly truncated. The IPHAS A star data are not compatible with models that propose a sudden change in metallicity at RG= 10 kpc.