Progressively more prolate dark matter halo in the outer Galaxy as traced by flaring HI gas

Understand

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.