Gaia ages show the Milky Way's disc flattened in two stages

Most stars in our galaxy sit in two overlapping discs: a thin, younger one and a thicker, older one. How they came about has been a long-running puzzle. A new study published in The Astrophysical Journal Letters offers what its authors call the first clear observational answer, the Instituto de Astrofísica de Canarias (IAC) announced on 19 October.

The question: shaken up or settled down?

Until now, observations fit two different stories equally well:

Telling them apart requires knowing how old stars are, especially the oldest ones, and that is notoriously hard.

How the team measured it

The researchers combined data from ESA's Gaia mission, which has measured the positions, motions and brightness of more than a billion stars, with ages from the ChronoGal project. ChronoGal, funded by the European Research Council and led by the IAC, delivers ages accurate to better than 5%, about 500 million years even for the oldest stars.

What they found

Younger stars sit closer to the galactic plane, but the change is not steady. There is a clear turning point around 10.8 billion years ago:

• Before it: in about 2 billion years, the disc went from over 4,100 light-years (1.26 kpc) thick to around 1,600 light-years (0.5 kpc).
• After it: thinning became very gradual, down to around 400 light-years (0.1 kpc) for stars forming today.

A change that fast cannot come from slow heating, the authors argue, especially in a young galaxy that was often colliding with others. It matches the upside-down picture instead: the thick disc formed in the turbulent early phase and the thin disc in the calmer phase that followed.

"This is the first clear observational evidence favouring one of the two models for the Galaxy's early stage," said lead author David Mirabal, a pre-doctoral researcher at the IAC and the University of La Laguna.

Why it matters

The result gives simulations of galaxy formation a hard number to match, and shows how much precise stellar ages add to Gaia's map. It also caps a busy month for European space science, after ESA's Euclid telescope found its first new comet. To see the Milky Way's band yourself, a dark site and our stargazing binoculars guide are a good start, and these phone tips help you photograph it.