The constraining effect of gas and the dark matter halo on the vertical stellar distribution of the Milky Way

TL;DR

This study models the Milky Way's vertical stellar distribution considering stars, gas, and dark matter, revealing how the dark matter halo constrains disk thickness and causes flaring at large radii.

Contribution

It provides a self-consistent three-component model of the galactic disk, highlighting the significant constraining effect of gas and dark matter on stellar vertical distribution.

Findings

Disk thickness increases with radius, especially beyond 17 kpc.

Dark matter halo reduces outer disk thickness by a factor of 3-4.

Including dark matter is essential for accurate outer disk structure modeling.

Abstract

We study the vertical stellar distribution of the Milky Way thin disk in detail with particular focus on the outer disk. We treat the galactic disk as a gravitationally coupled, three-component system consisting of stars, atomic hydrogen gas, and molecular hydrogen gas in the gravitational field of the dark matter halo. The self-consistent vertical distribution for stars and gas in such a realistic system is obtained for radii between 4-22 kpc. The inclusion of an additional gravitating component constrains the vertical stellar distribution toward the mid-plane, so that the mid-plane density is higher, the disk thickness is reduced, and the vertical density profile is steeper than in the one-component, isothermal, stars-alone case. We show that the stellar distribution is constrained mainly by the gravitational field of gas and dark matter halo in the inner and the outer Galaxy, respectively. We find that the thickness of the stellar disk (measured as the HWHM of the vertical density distribution) increases with radius, flaring steeply beyond R=17 kpc. The disk thickness is reduced by a factor of 3-4 in the outer Galaxy as a result of the gravitational field of the halo, which may help the disk resist distortion at large radii. The disk would flare even more if the effect of dark matter halo were not taken into account. Thus it is crucially important to include the effect of the dark matter halo when determining the vertical structure and dynamics of a galactic disk in the outer region.