Milky Way's Disk Grew Upside Down, With Younger Stars Forming in Ever-Thinner Layers

Start with the structure of the galaxy we live in. The Milky Way holds hundreds of billions of stars in a spiral pattern, and beneath that familiar shape it divides into a bulge, a disk and a halo. The disk itself comes in two parts, a thin disk and a thick disk, and a new study in The Astrophysical Journal Letters argues it can now say which one came first and how.

The two disks differ in more than shape. The thin disk holds young stars and middle-aged ones like our sun, along with the spiral arms where the solar system sits and most of the galaxy's gas and dust. It is where new stars form, and it has higher metallicity, enriched by earlier generations of stars that lived and died. The thick disk is puffier and more diffuse. It holds ancient stars, almost no new ones form there, and its metallicity is much lower. Astronomers sometimes call it a stellar fossil record.

For years, two explanations have competed. In one, stars are born in the thin disk and later get knocked away from its plane by encounters with gas clouds, the spiral arms or the central bar, a process called dynamical scattering. In the other, stars form in the inner disk while their parent gas settles into an ever thinner layer, so each generation is born in a flatter disk than the one before. Telling them apart requires accurate ages for individual stars, which is hard, especially for the oldest.

The paper is titled "A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages." Its lead author is David Mirabal, a predoctoral researcher at the Instituto de Astrofisica de Canarias. The team worked from Gaia Data Release 3, published in June 2022, and this is one of several papers from a collaboration aimed at pinning down stellar ages in both disks. They fit color-magnitude diagrams to the Gaia data, derived age-metallicity distributions for the stellar populations, and studied the vertical profile, meaning where in the disk those stars sit.

The central finding is a break about 10.85 billion years ago, a couple of billion years after the Big Bang. Earlier, the gas was turbulent and the galaxy was puffed up vertically, almost like a single thick disk, and stars were born orbiting within it. As the gas calmed, cooled and contracted, a thinner disk formed inside the thick one, while the older stars kept their orbits in the thick disk.

The numbers show how abrupt the change was. Before the break, the vertical scale height fell from over 1.26 kiloparsecs to roughly 0.5 in only 2 billion years. In round figures, the disk thinned from about 4,100 light-years thick to about 1,600. After that, thinning was far more gradual, and the youngest populations have a scale height of about 0.1 kiloparsecs, or roughly 400 light-years.

That favors the second explanation, an upside-down growth in which the disk built inward from a thick layer. On this reading, the thick disk's ancient stars were not scattered there by gas clouds or spiral arms. They were born there, when the disk was much thicker. The authors describe two distinct phases: an initial, highly turbulent epoch that produced the thick disk, followed by a quieter period of gradual gas settling that formed the thin disk. The result is today's split, with older, metal-poor stars and almost no star-forming gas in the thick disk, and younger stars, most star formation and most of the gas in the thin one.

Mirabal said in a press release that the result resolves a classic problem. Simulations allowed both formation models, he said, but observations had not decided between them. He called this the first clear observational evidence favoring one model for the galaxy's early stage, and credited the precision achieved for the older ages. The natural next step is testing the break against other stellar age datasets.