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

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 Ha Survey of the Northern Galactic Plane (IPHAS) photometry in the Galactic longitude range 160 degrees <l <200 degrees close to the equatorial plane (-1 degrees <b <+1 degrees). 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 similar to 100 Myr old. We find that the stellar density profile of young stars is well fit to an exponential with length-scale of (3020 +/- 120(statistical) +/- 180(systematic)) pc, which is comparable to that obtained in earlier studies, out to a Galactocentric radius of R(T) = (13.0 +/- 0.5(statistical) +/- 0.6(systematic)) kpc. At larger radii, the rate of decline appears to increase with the scalelength dropping to (1200 +/- 300(statistical) +/- 70(systematic)) 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 R(G) = 10 kpc.