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

TL;DR

This study uses 2MASS star counts and a stellar population model to constrain the structure and warp of the Milky Way's external disc, revealing asymmetries and differences between stellar, gas, and dust warps.

Contribution

It provides new constraints on the Milky Way's external disc parameters and characterizes the warp shape using near-infrared data and modeling.

Findings

The stellar disc is asymmetrical, similar to HI observations.

The warp on the positive longitude side is modeled as an S shape with a smaller slope than HI.

The negative longitude side shows peculiarities not captured by simple models.

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 and 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 Besancon, to constrain the external disc parameters such as its scale length, its cutoff radius, and the slope of the warp. In order to properly interpret the observations, the simulated stars are reddened using a three dimensional 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 a S shape warp but with a slope significantly smaller than the slope seen in the HI warp. At negative longitudes, the disc presents peculiarities which are not well reproduced by any simple model. Finally, comparing with the warp seen in the dust, it seems to follow a slope intermediate between the gas and the stars.