What's Up in the Milky Way? The Orientation of the Disc Relative to the Triaxial Halo

TL;DR

This study investigates whether the Milky Way's disc can be aligned with the halo's intermediate axis, finding it unlikely and suggesting the disc is tilted relative to the halo's principal axes, which has implications for testing alternative gravity theories.

Contribution

The paper demonstrates that stable intermediate-axis orientation of the Milky Way's disc is unlikely, proposing instead a tilted disc configuration relative to the halo's principal axes.

Findings

Disc cannot stably align with the halo's intermediate axis.

Simulations show the potential is rounder than Milky Way models in the Sagittarius Stream region.

The Milky Way's disc is likely tilted relative to the halo's principal axes.

Abstract

Models of the Sagittarius Stream have consistently found that the Milky Way disc is oriented such that its short axis is along the intermediate axis of the triaxial dark matter halo. We attempt to build models of disc galaxies in such an `intermediate-axis orientation'. We do this with three models. In the first two cases we simply rigidly grow a disc in a triaxial halo such that the disc ends up perpendicular to the global intermediate axis. We also attempt to coax a disc to form in an intermediate-axis orientation by producing a gas+dark matter triaxial system with gas angular momentum about the intermediate axis. In all cases we fail to produce systems which remain with stellar angular momentum aligned with the halo's intermediate axis, even when the disc's potential flattens the inner halo such that the disc is everywhere perpendicular to the halo's local minor axis. For one of these unstable simulations we show that the potential is even rounder than the models of the Milky Way potential in the region probed by the Sagittarius Stream. We conclude that the Milky Way's disc is very unlikely to be in an intermediate axis orientation. However we find that a disc can persist off one of the principal planes of the potential. We propose that the disc of the Milky Way must be tilted relative to the principal axes of the dark matter halo. Direct confirmation of this prediction would constitute a critical test of Modified Newtonian Dynamics.