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