A Twisted, Two-Break Milky Way Halo: What DESI Reveals
Using new data from the Dark Energy Spectroscopic Instrument (DESI), first author Songting Li and collaborators set out to map the shape and structure of the Milky Way’s stellar halo, the sparse, ancient population of stars that surrounds our Galaxy. Although this halo makes up only a tiny fraction of the Milky Way’s total stars, astronomers believe it carries a fossil record of the Galaxy’s past, including dramatic collisions with other galaxies. To trace that history, the team used K-giant stars: bright, evolved stars that can be seen at great distances, up to nearly 200,000 light-years from the Galactic center.
Sample Selection and Corrections
The authors began by selecting roughly 28,000 halo K giants from DESI’s latest Milky Way Survey observations. They filtered out stars belonging to the disk, nearby star clusters, and the Sagittarius tidal stream so that the remaining stars would better represent the true halo population. They then accounted for where DESI has and has not observed stars on the sky, as well as how far dimmer stars can be seen, important corrections to avoid being fooled by gaps or limits in the data.
Global Structure of the Halo
With their carefully corrected sample, the team modeled the halo as a triaxial ellipsoid, essentially a stretched-out 3D oval, whose density changes with distance. They found that the halo is tilted about 44° relative to the Milky Way’s disk, with axis ratios close to 10:8:7, meaning the structure is almost “prolate,” or football-shaped. More surprisingly, its radial density is not smooth: instead, it follows a triple power-law with two distinct break radii at about 16 kpc and 76 kpc (roughly 50,000 and 250,000 light-years). These “breaks” mark where the density of stars suddenly changes slope, revealing scars from the Galaxy’s biggest mergers. The inner break likely records debris from the Gaia-Sausage/Enceladus event, while the outer break may reflect the influence of the Large Magellanic Cloud, a massive companion galaxy still tugging on our halo today.
A Twisted and Changing Halo Shape
The models show that the inner halo (inside ~30 kpc) is oblate and mostly aligned with the disk, like a slightly squashed sphere, while the outer halo becomes more prolate and rotates toward being perpendicular to the disk. In fact, the major axis of the outer halo lines up with the “Vast Polar Structure” of satellite galaxies and star streams. Li and collaborators propose that this twisting shape may have formed when the Milky Way’s angular momentum shifted during a past interaction with a massive infalling satellite.
Local Overdensities and Wake Features
Because the stellar halo is far from smooth, the authors also mapped out anisotropies, places where the halo is unusually dense. They successfully re-detected well-known overdensities such as Hercules–Aquila (North and South) and the Virgo overdensity, each with their own smaller-scale break radii. They also identified features related to the LMC’s density wake, including a strong overdensity in the Pisces region and another in the northern Galactic cap showing a peak near 90 kpc, offering new observational evidence of the LMC’s ongoing gravitational disturbance.
Chemical Clues to Galactic History
Finally, the team found that more metal-poor stars (those with less iron relative to hydrogen) tend to be spread farther into the halo than metal-rich ones. This supports the idea that the oldest and most primitive stars were brought in by early cosmic collisions and flung to great distances, while later accretion events built the inner halo. Overall, DESI’s new map paints the Milky Way’s halo as a twisted, dynamically evolving structure, shaped by both ancient and still-unfolding interactions in our cosmic neighborhood.
Source: Li
