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

TL;DR

This study models the flaring of atomic hydrogen gas in the Milky Way to reveal that its dark matter halo is prolate and becomes more elongated with radius, reaching an axis ratio of 2.0 at 24 kpc.

Contribution

It provides the first evidence that the Milky Way's dark matter halo is prolate and increasingly elongated at larger radii based on gas flaring observations.

Findings

Dark matter halo is prolate in shape.

Halo becomes more prolate at larger radii.

Prolate halo explains long-lived galactic warps.

Abstract

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.