The Milky Way's external disc constrained by 2MASS star counts

¹ Observatoire de Besançon, Institut UTINAM, Université de Franche-Comté, BP 1615, 25010 Besançon Cedex, France e-mail: celine@obs-besancon.fr
² Département de Physique et Centre de Recherche en Astrophysique de Québec, Université Laval, Québec, G1K 7P4, Canada 


Received: 13 November 2008
Accepted: 13 December 2008

Abstract

Context. Thanks to recent large-scale surveys in the near infrared, such as 2MASS, the galactic plane that most suffers from extinction is revealed so its overall structure can be studied.

Aims. This work aims at constraining the structure of the Milky Way external disc as seen in 2MASS data, and in particular the warp.

Methods. We use the Two Micron All Sky Survey (hereafter 2MASS) along with the Stellar Population Synthesis Model of the Galaxy, developed in Besançon, to constrain the external disc parameters, such as its scale length, its cutoff radius, and the slope of the warp. To properly interpret the observations, the simulated stars are reddened using a 3D extinction map. The shape of the stellar warp is then compared with previous results and with similar structures in gas and dust.

Results. We find new constraints on the stellar disc, which is shown to be asymmetrical, similar to observations of HI. The positive longitude side is found to be easily modelled with S-shaped warp but with a significantly smaller slope than the one seen in the HI warp. At negative longitudes, the disc presents peculiarities that are not reproduced well by any simple model. Finally, comparing with the warp seen in the dust, it seems to follow an intermediate slope between the gas and the stars.