The Milky Way left its “poor old heart” in and around the constellation Sagittarius, astronomers report. New data from the Gaia spacecraft reveal the full extent of what appears to be the galaxy’s original core, the ancient stellar population around which the rest of the Milky Way grew, coming together more than 12.5 billion years ago. .
“People have long speculated that such a large population [of old stars] they should exist in the center of our Milky Way, and Gaia now shows that they are there,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.
The ancient heart of the Milky Way is a round protogalaxy that spans nearly 18,000 light-years and has approximately 100 million times the mass of the sun in the starsor about 0.2 percent of the Milky Way’s current stellar mass, Rix and colleagues report in a study published September 7 at arXiv.org.
“This study really helps to firm up our understanding of this very, very, very young stage in the life of the Milky Way,” says Vasily Belokurov, an astronomer at the University of Cambridge who was not involved in the work. “Not much is really known about this period of the Milky Way’s life,” he says. “We’ve seen glimpses of this population before,” but the new study offers “a bird’s-eye view of the entire structure.”
Most of the stars in the central region of the Milky Way are metal-rich, because the stars originated in a populated metropolis that previous stellar generations had enriched with those metals through supernova explosions. But Rix and his colleagues wanted to find the exceptions to the rule, stars so metal-poor that they must have been born long before the rest of the galaxy’s stellar denizens appeared, what Rix calls “a needle-point exercise.” haystack”. ”
His team turned to data from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way (Serial number: 06/13/22). The astronomers searched for about 2 million stars within a wide region around the center of the galaxy, which lies in the constellation of Sagittarius, looking for stars with metal-to-hydrogen ratios of no more than 3 percent of the sun’s.
Astronomers then examined how those stars move through space, retaining only the ones that don’t get flung out into the vast halo of metal-poor stars that shroud the Milky Way’s disk. The end result: a sample of 18,000 ancient stars that represents the core around which the entire galaxy flourished, the researchers say. Taking stars obscured by dust into account, Rix estimates that the protogalaxy is between 50 and 200 million times the mass of the Sun.
“That’s the original core,” says Rix, and it houses the oldest stars in the Milky Way, which he says are probably older than 12.5 billion years. The protogalaxy was formed when several large groups of stars and gas conglomerated long ago, before the first disk of the Milky Way, the so-called thick disc — arose (Serial number: 03/23/22).
The protogalaxy is compact, meaning that little has disturbed it since its formation. Smaller galaxies have slammed into the Milky Way, increasing its mass, but “we didn’t have subsequent mergers that went deep into the core and shook it up, because then the core would be bigger now,” says Rix.
The new data on the protogalaxy even captures the Milky Way’s initial spin: its transition from an object that didn’t spin to one that now does. The oldest stars in the proto-Milky Way barely rotate around the galactic center, instead dipping in and out of it, while slightly younger stars show more and more motion around the galactic center. “This is the Milky Way trying to become a disk galaxy,” says Belokurov, who saw the same twist in research he and a colleague reported in July.
Today, the Milky Way is a giant galaxy that rotates rapidly: every hour, our solar system travels 900,000 kilometers of space as we race around the center of the galaxy. But the new study shows that the Milky Way began as a modest protogalaxy whose stars still shine today, stars that astronomers can now scrutinize for more clues about the galaxy’s birth and early evolution.