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Gaia finds parts of the Milky Way much older than expected

on 25 March 2022

Using data from ESA’s Gaia mission astronomers have shown that a part of the Milky Way known as the ‘thick disc’ began forming 13 billion years ago, around 2 billion years earlier than expected, and just 0.8 billion years after the Big Bang.

This surprising result comes from an analysis performed by researchers from the Max-Planck Institute for Astronomy, Heidelberg, Germany. They took brightness and positional data from Gaia’s Early Data Release 3 (EDR3) dataset and combined it with measurements of the stars’ chemical compositions, as given by data from China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) for roughly 250 000 stars to derive their ages.

The age of a star is one of the most difficult parameters to determine. It cannot be measured directly but must be inferred by comparing a star’s characteristics with computer models of stellar evolution. The compositional data helps with this. The Universe was born with almost exclusively hydrogen and helium. The other chemical elements, known collectively as metals to astronomers, are made inside stars, and exploded back into space at the end of a star’s life, where they can be incorporated into the next generation of stars. So, older stars have fewer metals and are said to have lower metallicity.

The LAMOST data gives the metallicity. Together, the brightness and metallicity allow astronomers to extract the star’s age from the computer models. Before Gaia, astronomers were routinely working with uncertainties of 20-40 percent, which could result in the determined ages being imprecise by a billion years or more.

Gaia’s EDR3 data release changes this. “With Gaia’s brightness data, we are able to determine the age of a sub giant star to a few percent,” says Maosheng Xiang, one of the researchers. Armed with precise ages for a quarter of a million sub giant stars spread throughout the galaxy, the research team began the analysis.

2022 03 Gaia disc Image B

Milky Way edge-on view

The stellar ages clearly revealed that the formation of the Milky Way fell into two distinct phases. In the first phase, starting just 0.8 billion years after the Big Bang, the thick disc began forming stars. The inner parts of the halo may also have begun to come together at this stage, but the process rapidly accelerated to completion about two billion years later when a dwarf galaxy known as Gaia-Sausage-Enceladus merged with the Milky Way. It filled the halo with stars and, as clearly revealed by the new work, triggered the nascent thick disc to form the majority of its stars. The thin disc of stars which holds the Sun, was formed during the subsequent, second phase of the galaxy’s formation.

New observations could come in the near future as the James Webb Space Telescope has been optimised to see the earliest Milky Way-like galaxies in the Universe. And on 13 June this year, Gaia will release its full third data release (Gaia DR3). This catalogue will include spectra and derived information like ages and metallicity, making studies like Maosheng’s even easier to conduct.

In Romania, the Astronomical Institute of the Romanian Academy is involved in four working groups of the CU9 (Coordination Unit 9) with regards to access to the GAIA catalogue, in the global GAIA-FUN-SSO network for tracking Solar System objects discovered by the Gaia Mission and in the Gaia Science Alert working group.

More details can be found at this link.

Credit: (1) ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Moitinho. (2) Stefan Payne-Wardenaar / MPIA