According to scientists, the Milky Way left its "poor old heart" in and around the constellation Sagittarius. New data from the Gaia spacecraft reveal the whole extent of what appears to be the galaxy's original nucleus — the ancient stellar population that the rest of the Milky Way grew around — which came together more than 12.5 billion years ago.
“Many people have long assumed that such a large number [of old stars] might exist in the center of our Milky Way, and Gaia now shows that there are,” according to astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.
Rix and colleagues describe the Milky Way's ancient heart as a round protogalaxy that covers roughly 18,000 light years and has roughly 100 million times the sun's current stellar mass, or about 0.2% of the Milky Way's current stellar mass, according to a study published on September 7 at arXiv.org.
Vasily Belokurov, an astronomer at the University of Cambridge who was not involved in the study, says the new study "provides a bird's-eye view of the whole structure."
Because most stars in the Milky Way's central region were enriched with metals due to supernova explosions, Rix and his colleagues sought out the exceptions to the rule, stars that were so metal-poor they must have been born before the rest of the galaxy's stellar inhabitants — a "needle-in-a-haystack exercise."
His team consulted data from the Gaia spacecraft, which was launched in 2013 on a mission to survey the Milky Way (SN: 6/13/22). The astronomers searched for 2 million stars in a broad area near the galaxy's center, which is in the constellation Sagittarius, searching for stars with metal-to-hydrogen ratios no more than 3 percent of the sun's gravity.
The astronomers then examined how these stars move through space, retaining only the ones that don't dart off into the vast halo of metal-poor stars that engulf the Milky Way's disk. The end result: a sample of 18,000 ancient stars that represents the kernel around which the entire galaxy grew, according to Rix.
"That's the original core," Rix asserts, and it holds the Milky Way's oldest stars, which he believes are older than 12.5 billion years, before the Milky Way's first disk — the so-called thick disk — arose (SN: 3/23/22).
The protogalaxy is small, which means that little has changed since its formation. Smaller galaxies have crashed into the Milky Way, increasing its mass, but "we didn't have any later mergers that deeply penetrated into the core and shaken it up, because then the core would be larger now."
Belokurov and a colleague described the Milky Way's initial spin-up in their latest findings. The oldest stars in the proto–Milky Way barely revolve around the galaxy's center but instead dive in and out of it.
The Milky Way today is a gigantic galaxy that spins at an incredible rate — each hour our solar system travels through 900,000 kilometers of space as we approach the galaxy's center. However, the new study suggests that the Milky Way started as a modest protogalaxy that today is capable of discovering additional information about the galaxy's origin and early evolution.