A galactic disk in a spiral galaxy is generally believed to be embedded in anextended 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 thedark matter halo in the Milky Way Galaxy is prolate in shape. Further, it isincreasingly more prolate at larger radii, with the vertical-to-planar axisratio monotonically increasing to 2.0 at 24 kpc. This is obtained by modelingthe observed steeply flaring atomic hydrogen gas layer in the outer Galacticdisk, where the gas is supported by pressure against the net gravitationalfield of the disk and the halo. The resulting prolate-shaped halo can explainseveral long-standing puzzles in galactic dynamics, for example, it permitslong-lived warps thus explaining their ubiquitous nature.
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