Milky Way's Disk Grew 'Upside Down,' With Younger Stars Forming in Ever-Thinner Layers
Gaia ages for stars across the galaxy show the disk flattened fast until about 10.8 billion years ago, then slowly, favoring one of two rival models of how it formed.
The Milky Way's disk was built from the thick layer inward, an "upside-down" formation in which each new generation of stars was born in a thinner disk than the one before, according to a new study of Gaia data in The Astrophysical Journal Letters. The result favors one of two competing explanations astronomers have debated for years.
The Milky Way has a bulge, a halo and a disk, and the disk itself is split in two. The thin disk holds young and middle-aged stars like the sun, the spiral arms where the solar system sits, and most of the galaxy's gas and dust. It is where new stars form. The thick disk is puffier and more diffuse, holds ancient, metal-poor stars, and sees almost no new star formation. Astronomers sometimes call it a stellar fossil record.
Why there are two disks has been an open question. One explanation says stars are born in the thin disk and later scattered away from its plane by encounters with gas clouds, spiral arms or the central bar. The other says the gas that formed the stars kept settling into an ever-thinner disk, so each generation was born closer to the plane. Telling them apart requires accurate ages for individual stars, which is hard, especially for the oldest.
The new paper, titled "A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages," was led by David Mirabal, a predoctoral researcher at the Instituto de Astrofísica de Canarias. It uses data from Gaia Data Release 3, published in June 2022, and fits stars on color-magnitude diagrams to derive age and metallicity distributions. The team also studied how those populations are spread vertically above and below the galactic plane.
Their central finding is a break about 10.8 billion years ago. Before then, the disk flattened extremely fast. "We find evidence that the scale height decrease rate changed drastically (~10.85) Gyr ago," the authors write. At older ages, the vertical scale height dropped from more than 1.26 kiloparsecs to roughly 0.5 kiloparsecs in about 2 billion years. In more familiar units, the disk thinned from about 4,100 light-years thick to about 1,600 light-years. After that, the thinning was gradual, reaching only about 400 light-years for the youngest stars.
That means ancient stars in the thick disk were not scattered there later. They were born there, when the whole disk was much thicker and the gas was turbulent. As the gas cooled and calmed, it shrank into a thinner disk inside the thick one. "The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed," the researchers write. They describe two phases: a highly turbulent early epoch that produced the thick disk, and a quieter period of gradual gas settling that produced the thin disk.
"This result resolves a classic problem in astrophysics," Mirabal said in a press release. "In the simulations, both disk formation models were possible, but there was a lack of observational evidence to determine which one occurred in the Milky Way. This is the first clear observational evidence favoring one of the two models for the galaxy's early stage." He said the precision achieved for the oldest stars' ages made it possible.
