Did the Milky Way Form Faster Than We Thought?

A study has found thousands of ancient stars on orbits like our Sun's. The find suggests that the Milky Way's thin disk — full of stars on orderly orbits around the galactic center — is much older than previously thought.

Stars are rather secretive about their age, just like their Hollywood counterparts. Especially when they’re lightweight and long-living, they keep appearances for most of their long lives. To estimate how old a star really is, astronomers have look for hidden signs, like the tiny amount of elements heavier than hydrogen and helium in their atmospheres, the so-called metals.

Metals are produced in massive stars and get recycled via supernovae into the interstellar medium, where they’re slowly built into the next stellar generation. The more metals a star contains (that is, the higher its metallicity), the later in time it was born — usually.

Reality, as usual, is more complicated. Metallicity increases consistently over time only within a reasonably coherent stellar population, such as a group of stars that enriches itself from a single gas cloud over generations. But the same isn’t necessarily the case between different populations.

“Stellar age is one of the most difficult things to measure,” says Samir Nepal (Leibniz Institute for Astrophysics in Potsdam, Germany). When he and his colleagues wanted to know the ages of no less than 200,000 stars within the Milky Way, they had to walk a different path.

Ancient Stars in the Thin Disk

The European Space Agency’s Gaia satellite provided the team with stars of precisely known distance, temperature, and metallicity. Using statistical tools and machine-learning techniques, Nepal and his team identified about 8,500 stars within Gaia’s catalog, most of them older than 10 billion years — double the age of our Sun. Some are even older than 13 billion years, which means that they formed less than 1 billion years after the Big Bang.

Surprisingly, these methusalems are relatively close to us, within 3,200 light-years of our solar system. They move around the galactic center in the thin disk — what we see as the faint band of light encircling the sky in clear, dark nights. This is the part of the galaxy that contains the most stars, dust, and gas.

But the thin disk is thought to be young, having formed only 8 to 10 billion years ago. Astronomers expected older stars to reside in the Milky Way’s more extended thick disk. So how stars as old as 13 billion years be in the thin disk?

“Our study suggests that the thin disk of the Milky Way may have formed much earlier than we had thought, and that its formation is strongly related to the early chemical enrichment of the innermost regions of our galaxy,” explains team member Cristina Chiappini (also at Potsdam).

What’s more, while some of the stars of the sample are metal-poor, as expected for stars formed early on in the universe, others have twice the metal content of our much younger Sun. Huge numbers of forming and exploding massive stars seem to have enriched the thin disk with metals very early on – proving the point that metallicity alone does not suffice to determine their ages.

“From a theoretical point of view, there is no objection to having an old star — of 10 billion years or more — with metallicity more than two times the Sun,” Nepal adds.

Connecting “Here and Now” to “Back Then”

According to Nepal, Chiappini, and their team, the Milky Way results are in line with observations by the James Webb Space Telescope (JWST) and the Atacama Large Millimeter Array. Astronomers have used these telescopes to find glowing disks in very distant galaxies, indicating organized rotation and massive star formation even in early times.

The JWST/ALMA galaxies are so far away that their light originated from earlier than 1 billion or so years after the Big Bang — about the same time the newfound ancient stars in the Milky Way’s thin disk would have been formed. So perhaps it shouldn’t be surprising that the Milky Way’s thin disk is younger than we thought.

There are caveats, however: “The cold disks that have been found in the early universe by JWST are typically much more massive than what we think the early Milky Way was like at that time,” comments Anke Ardern-Arentsen (University of Cambridge, UK), who investigates old stellar populations and the ancient Milky Way. “They would have evolved into much larger galaxies than the Milky Way by the present-day.”

Ardern-Arentsen, who was not involved in the study, thinks that the claim of a much older thin disk will require more evidence, for example, by comparing against galaxy simulations or achieving the same result using other datasets and methods. “If more evidence is found to support the very early disk scenario, this will have interesting implications for our galaxy formation models, although it won’t completely overturn them.”

Reference

Samir Nepal et al. "Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS", to appear in Astronomy & Astrophysics