Dating the Dawn of the Milky Way Disk

The iconic disks of spiral galaxies are well known across the local universe and now far beyond. A recent study looks at the Milky Way to determine when its own disk emerged.

Spinning into Spirals

The Milky Way, a grand spiral galaxy with stars and gas rotating in a disk, did not always look this way. Galaxies begin as chaotic clumps of gas and stars inside a knot of dark matter, and at some point in time, spiral galaxies must transform into the flat rotating disks we observe them as today. Recent JWST observations have uncovered disk galaxies emerging much earlier in the universe than previously thought, shifting our understanding of how quickly spiral galaxies come together and settle into disks. While we cannot study the detailed assembly history of these very distant galaxies, we can dissect galaxies in the local universe to build a better timeline of galaxy evolution.

Looking as close to home as possible, astronomers have used the kinematics and chemical makeup of stars in our own Milky Way to trace the moment when the galaxy started to host a rotating stellar disk. This period, known as the “spin-up,” marks when the bulk kinematic signature of the galaxy transitioned from one dominated by random motions to one dominated by a rotating disk.

Stars hold on to the kinematic and chemical information they are born with, so stars of different ages and metallicities across the galaxy should trace the development of the disk. Because stellar ages have been difficult to constrain observationally, studies of galactic spin-up have typically relied on readily available metallicity measurements as a proxy for age; however, metallicity is not a clear-cut age indicator, making it difficult to truly distinguish different stellar populations across the Milky Way’s disk. Luckily, new major surveys have recently provided stellar databases adding the stellar age measurements necessary to pin down when the Milky Way spun up.

Dawn of the Galactic Disk

Sofia Feltzing (Lund Observatory) and collaborators used a catalog of over 300,000 subgiant stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope with metallicities, kinematics, and crucial stellar ages to determine when the Milky Way began to host a stellar disk. After making quality cuts to ensure they included only the most robustly measured stars, the authors divided the sample into two distinct stellar populations that settled into the Milky Way’s disk differently: older stars with a high abundance of α elements (like oxygen, magnesium, and silicon) relative to iron and younger stars with low α-element abundances.

With these star samples, Feltzing and team analyzed how both stellar rotational velocity and age varied with metallicity to determine when the Milky Way spun up. They found that high-α stars clearly move from having random motions to being rotationally supported over a short range of metallicities about 12.5 billion years ago, while the low-α stars seem to begin in a rotating disk and stay there. When varying their selection criteria and looking at the full sample, the authors determined that the Milky Way spin-up happened between 12.1 to 12.5 billion years ago — the first time Milky Way spin-up has been age-dated in this way.

These results imply a rapid chemical evolution and change in kinematic properties as stars went from randomly orbiting to rotating in an ordered disk in the Milky Way. The authors suggest that future studies should focus on lower-metallicity stars to better disentangle stars in the disk from stars in the halo, which will lead to a more precise determination of the Milky Way’s spin-up. Upcoming large-scale surveys will make this possible and allow astronomers to build a better picture of galaxy formation in the early universe.

Citation

“Dawn of the Milky Way Disk: Determination of When a Rotationally Supported Disk Appears and Dating the Spin-up of the Disk,” Sofia Feltzing et al 2026 ApJL 1004 L28. doi:10.3847/2041-8213/ae6f14