An international team of researchers has obtained the largest set of detailed observations to date of the oldest stars in the center of our Milky Way galaxy. The Pristine Inner Galaxy Survey (PIGS) team finds that this group of stars is slowly rotating around the center of the Milky Way, even though it is believed to have formed in a chaotic manner. They also seem to spend most of their long lives near the galactic center. Dr Anke Arentsen, a member of the PIGS team from Cambridge University, is presenting the new work this week at the 2023 National Astronomy Meeting at Cardiff University. Some of the stars that were born in the first billion years after the Big Bang still exist today, and can be used to study what galaxies were like when they were just beginning to form. They can be recognized by their pristine chemical composition, made up mostly of hydrogen and helium, with a much lower abundance of heavier elements than younger stars like the Sun. Astronomers typically look for these ancient stars far from the plane of the Milky Way’s disk. , in the low-density halo surrounding our galaxy, where they are easier to find.

Galaxy formation models suggest that the oldest stars are expected to be present in the dense inner parts of the Milky Way. Finding them in this region is challenging as our line of sight to the center of the galaxy is blocked by vast amounts of interstellar dust, and old stars are extremely rare compared to the vast majority of their younger peers. In the PIGS project, Arentsen and his team used a special imaging filter on the Canada-France-Hawaii Telescope (CFHT) to efficiently shortlist candidate stars. They were confirmed with spectroscopic observations at the Anglo-Australian Telescope (AAT), resulting in the largest set yet of detailed observations of pristine stars in the galaxy’s interior. The PIGS observations were then combined with data from the Gaia space mission to study how these ancient stars move through the Milky Way. It turns out that the older the stars, the more chaotic their motions, but even the oldest stars found still show an average rotation around the center of the galaxy. They also show that many of these stars spend most of their lives in the inner Galaxy, within a sphere that reaches only halfway between the galactic center and the Sun.