Title: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 40000 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.