The Structure of the Outer Galactic Disc as revealed by IPHAS early A Stars

TL;DR

This paper analyzes the density profile of young stars in the outer Galactic disc using IPHAS data, revealing an exponential decline with a potential truncation and challenging models with metallicity jumps.

Contribution

It provides new constraints on the Galactic disc's stellar density profile using a large sample of early A stars, refining previous models and ruling out certain metallicity change hypotheses.

Findings

The stellar density profile follows an exponential with a 3020 pc scale length.

The profile declines more steeply beyond 13 kpc, with a 1200 pc scale length.

Data do not support models with a sudden metallicity change at 10 kpc.

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 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 parameterised 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 Myrs old. We find the stellar density profile of young stars is well fit to an exponential with length scale of (3020 \pm 120_{statistical} \pm 180_{systematic}) pc, which is comparable to that obtained in earlier studies, out to a Galactocentric radius of R_T = (13.0 \pm 0.5_{statistical} \pm 0.6_{systematic}) kpc. At larger radii the rate of decline appears to increase with the scale length dropping to (1200 \pm 300_{statistical} \pm 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.