Extremely Flat Haloes and the Shape of the Galaxy

TL;DR

This paper introduces highly flattened galaxy models with constant rotation curves, explores their properties, and constrains the shape of the Milky Way's dark halo, finding it cannot be flatter than E4 based on observational data.

Contribution

The study develops a new class of flattened galaxy models and applies them to constrain the Milky Way's dark halo shape using kinematic data.

Findings

Dark halo cannot be flatter than E4 (q ~ 0.57) at the Solar circle.

Models with q less than 0.57 fail to match in-plane and out-of-plane constraints.

Highly flattened models are inconsistent with observed stellar and satellite kinematics.

Abstract

We present a set of highly flattened galaxy models with asymptotically constant rotation curves. The mass density in the equatorial plane falls like (distance)$^{-1}$ at large radii. Although the inner equidensity contours may be spherical, oblate or prolate, the outer parts are always severely flattened. The elongated shape is supported by rotation or tangential velocity anisotropy. The models are thickened Mestel discs, and form a previously undiscovered part of the Miyamoto & Nagai sequence of flattened galaxies. The properties of the models -- axis ratios, velocity dispersions, streaming velocities and distribution functions -- are all discussed in some detail. We pose the question: are extremely flattened or disk-like haloes possible for the Milky Way galaxy? This has never been examined before, as very flattened halo models were not available. We fit the rotation curve and the vertical kinematics of disc stars in the solar neighbourhood to constrain the overall shape of the Galaxy. Denoting the ratio of polar axis to major axis by $q$, we show that models with $q\lesssim 0.57$ cannot simultaneously reproduce the in-plane and out-of-plane constraints. The kinematics of the Sagittarius galaxy also strongly disfavour models with high flattening, as the orbital plane precession is too great and the height reached above the Galactic plane is too small. At least for our Galaxy, the dark halo cannot be flatter than E4 (or axis ratio $q \sim 0.57$) at the Solar circle. Models in which the dark matter is accounted for by a massive baryonic disc or by decaying neutrinos are therefore ruled out by constraints from the rotation curve and the vertical kinematics.