Modelling star counts in the Monoceros stream and the Galactic anti-centre

Context. There is a continuing debate as to the form of the outer disc of the Milky Way galaxy, which has important implications for understanding the Galaxy’s formation. Stars are known to exist at a galacto-centric distance of at least 20 kpc. However, there is much debate as to whether these stars can be explained as part of the disc or whether another extragalactic structure, the so-called Monoceros ring/stream, is required Aims. We examine the outer disc of the Galaxy toward the anti-centre to determine whether the star counts can be explained by the thin and thick discs alone. Methods. By using Sloan star counts and extracting the late F and early G dwarfs, it is possible to directly determine the density of stars out to a galacto-centric distance of about 25 kpc. These are then compared with a simple flared-disc model. Results. A flared-disc model is shown to reproduce the counts along the lines of sight examined, if the thick disc does not have a sharp cut off. The flare starts at a Galacto-centric radius of 16 kpc and has a scale length of 4.5 ± 1.5 kpc Conclusions. Whilst the interpretation of the counts in terms of a ring/stream cannot be definitely discounted, it does not appear to be necessary, at least along the lines of sight examined towards the anti centre.