Progressively more prolate dark matter halo in the outer Galaxy as
  traced by flaring HI gas
  A galactic disk in a spiral galaxy is generally believed to be embedded in an
extended dark matter halo, which dominates its dynamics in the outer parts.
However, the shape of the halo is not clearly understood. Here we show that the
dark matter halo in the Milky Way Galaxy is prolate in shape. Further, it is
increasingly more prolate at larger radii, with the vertical-to-planar axis
ratio monotonically increasing to 2.0 at 24 kpc. This is obtained by modeling
the observed steeply flaring atomic hydrogen gas layer in the outer Galactic
disk, where the gas is supported by pressure against the net gravitational
field of the disk and the halo. The resulting prolate-shaped halo can explain
several long-standing puzzles in galactic dynamics, for example, it permits
long-lived warps thus explaining their ubiquitous nature.
