The Milky Way has left its “poor old heart” in and around the constellation Sagittarius, astronomers report. New data from the Gaia spacecraft reveals 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 – which gathered there is over 12.5 billion years old.
“People have long speculated that such a large population [of old stars] should exist at the center of our Milky Way, and Gaia is now showing that they are there,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.
The ancient core of the Milky Way is a round protogalaxy that spans nearly 18,000 light-years and has about 100 million times the mass of the sun in the starsor about 0.2% of the current stellar mass of the Milky Way, report Rix and his colleagues in a study published September 7 on arXiv.org.
“This study really helps solidify 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 known about this period in the life of the Milky Way,” he says. “We’ve seen glimpses of this population before,” but the new study gives “an overview of the whole structure.”
Most stars in the central region of the Milky Way are teeming with metals, because the stars originated in a crowded metropolis that previous stellar generations had enriched with these metals through supernova explosions. But Rix and his colleagues wanted to find the exceptions to the rule, stars so metal-poor they must have been born long before the rest of the galaxy’s stellar inhabitants arrived – what Rix calls “a needle exercise in a haystack”. ”
His team turned to data from the Gaia spacecraft, which launched in 2013 on a mission to map the Milky Way (SN: 06/13/22). Astronomers have searched about 2 million stars in a vast region around the center of the galaxy, which lies in the constellation Sagittarius, looking for stars with a metal-to-hydrogen ratio no more than 3% of that of the Sun.
Astronomers then examined how these stars move through space, picking out only those that don’t soar into the vast halo of metal-poor stars engulfing 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. Factoring in stars obscured by dust, Rix estimates that the protogalaxy is between 50 million and 200 million times more massive than the sun.
“It’s the original core,” Rix says, and it’s home to the oldest stars in the Milky Way, which he says probably have ages greater than 12.5 billion years. The protogalaxy formed when several large clusters of stars and gas coalesced long ago, before the Milky Way’s first disk – the so-called thick disc – was born (SN: 03/23/22).
The protogalaxy is compact, which means that little has disturbed it since its formation. Smaller galaxies crashed into the Milky Way, increasing its mass, but “we didn’t have subsequent mergers that went deep into the core and shook it, because then the core would be bigger now”, Rix said.
The new protogalaxy data even captures the Milky Way’s initial rotation — its transition from an object that didn’t spin to one that now does. The oldest stars in the proto-Milky Way barely orbit around the center of the galaxy, but instead dip into it, while slightly younger stars show increasing movement around the galactic center. “It’s the Milky Way trying to become a disk galaxy,” says Belokurov, who has seen the same spin-up in research he and a colleague reported in July.
Today, the Milky Way is a giant, rapidly rotating galaxy – 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 bright today, stars that astronomers can now peer into for further clues about the galaxy’s birth and early evolution. .