A Vertically Orientated Dark Matter Halo Marks a Flip of the Galactic Disk

Abstract

Unveiling the 3D shape of the Milky Way's dark-matter halo is critical to understanding its formation history. We created an innovative dynamical model with minimal assumptions on the internal dynamical structures and accommodates a highly flexible triaxial DM halo. By applying the method to 6D phase-space data of K-giant stars from LAMOST + Gaia, we robustly determine the 3D dark-matter distribution of the Milky Way out to approximately $50$ kpc. We discover a triaxial, nearly oblate dark-matter halo with $q_{\mathrm{DM}}=Z/X=0.92\pm 0.08$, $p_{\mathrm{DM}}=Y/X=0.8\pm 0.2$ averagely within 50 kpc, where $Z$ axis is defined perpendicular to the stellar disk. The axes ratio $q_{\mathrm{DM}}>p_{\mathrm{DM}}$ is strongly preferred; the long-intermediate axis plane of the dark-matter halo is unexpectedly vertical to the Galactic disk, yet aligned with the `plane of satellites'. This striking configuration suggests that the Galactic disk (and the inner halo) has flipped, likely torqued by minor mergers, from an original alignment with the outer dark-matter halo and satellite plane, as supported by Milky Way analogues from Auriga and TNG50. By allowing $q_{\mathrm{DM}}(r)$ and $p_{\mathrm{DM}}(r)$ vary with radii, we find tentative evidence that the dark-matter halo is twisted, that it agrees alignment with the disk in the inner regions and transitions to a vertical orientation at $r\gtrsim 20$ kpc, supporting the disk flip scenario prediction. Such disk reorientation is non-trivial yet its physical mechanism is straightforward to comprehend and naturally originates a vertical satellite plane. Our findings offer a unified framework that links dark-matter halo orientation, satellite alignment, and disk evolution, reinforcing the internal consistency of the Milky Way in $\Lambda$CDM model.

16 pages

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