Flaring and warping of the Milky Way disk: not only in the gas

TL;DR

This study uses 2MASS data to demonstrate that the Milky Way's stellar disk exhibits flaring and warping in the outer regions, similar to the gas disk, affecting star density distribution.

Contribution

It provides evidence that stellar disk flaring and warping are significant in the outer Milky Way, challenging the need for a disk cut-off and clarifying the relation to the thick disk.

Findings

Stellar disk thickens with increasing distance from the Sun.

Warp asymmetry is observed in the outer disk.

Flaring explains the star density drop near the Galactic plane.

Abstract

This paper presents an investigation of the outer disk structure by using data from a recent release of the 2 micron sky survey (2MASS). This 2MASS data show unambiguously that the stellar disk thickens with increasing distance from the Sun. In one of the field (longitude l$=$240) there is also strong evidence of an assymetry associated with the Galactic warp. This flaring and warping of the stellar disk is very similar to the features observed in the HI disk. The thickening of the stellar disk explains the drop in density observed near the Galactic plane: stars located at lower Galactic latitudes are re-distributed to higher latitudes. It is no longer necessary to introduce a disk cut-off to explain the drop in density. It is also not clear how this flaring disk is distinct from the thick disk in the outer disk region. At least, for lines of sight in the direction of the outer disk, the thickening of the disk is sufficient to account for the excess in star counts attributed by some models to the thick disk.