The Milky Method left its “poor previous coronary heart” in and across the constellation Sagittarius, astronomers report. New knowledge from the Gaia spacecraft reveal the total extent of what appears to be the galaxy’s unique nucleus — the traditional stellar inhabitants that the remainder of the Milky Method grew round — which got here collectively greater than 12.5 billion years in the past.
“Folks have lengthy speculated that such an unlimited inhabitants [of old stars] ought to exist within the heart of our Milky Method, and Gaia now reveals that there they’re,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.
The Milky Method’s historical coronary heart is a spherical protogalaxy that spans practically 18,000 light-years and possesses roughly 100 million instances the mass of the solar in stars, or about 0.2 % of the Milky Method’s present stellar mass, Rix and colleagues report in a examine posted September 7 at arXiv.org.
“This examine actually helps to agency up our understanding of this very, very, very younger stage within the Milky Method’s life,” says Vasily Belokurov, an astronomer on the College of Cambridge who was not concerned within the work. “Not a lot is absolutely recognized about this era of the Milky Method’s life,” he says. “We’ve seen glimpses of this inhabitants earlier than,” however the brand new examine offers “a chicken’s-eye view of the entire construction.”
Most stars within the Milky Method’s central area abound with metals, as a result of the celebrities originated in a crowded metropolis that earlier stellar generations had enriched with these metals via supernova explosions. However Rix and his colleagues wished to seek out the exceptions to the rule, stars so metal-poor they should have been born nicely earlier than the remainder of the galaxy’s stellar denizens got here alongside — what Rix calls “a needle-in-a-haystack train.”
His staff turned to knowledge from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Method (SN: 6/13/22). The astronomers searched about 2 million stars inside a broad area across the galaxy’s heart, which lies within the constellation Sagittarius, on the lookout for stars with metal-to-hydrogen ratios not more than 3 % of the solar’s.
The astronomers then examined how these stars transfer via area, retaining solely those that don’t dart off into the huge halo of metal-poor stars engulfing the Milky Method’s disk. The top end result: a pattern of 18,000 historical stars that represents the kernel round which the complete galaxy blossomed, the researchers say. By accounting for stars obscured by mud, Rix estimates that the protogalaxy is between 50 million and 200 million instances as huge because the solar.
“That’s the unique core,” Rix says, and it harbors the Milky Method’s oldest stars, which he says most likely have ages exceeding 12.5 billion years. The protogalaxy shaped when a number of giant clumps of stars and fuel conglomerated way back, earlier than the Milky Method’s first disk — the so-called thick disk — arose (SN: 3/23/22).
The protogalaxy is compact, which implies little has disturbed it since its formation. Smaller galaxies have crashed into the Milky Method, augmenting its mass, however “we didn’t have any later mergers that deeply penetrated into the core and shook it up, as a result of then the core could be bigger now,” Rix says.
The brand new knowledge on the protogalaxy even seize the Milky Method’s preliminary spin-up — its transition from an object that didn’t rotate into one which now does. The oldest stars within the proto–Milky Method barely revolve across the galaxy’s heart however dive out and in of it as an alternative, whereas barely youthful stars present increasingly motion across the galactic heart. “That is the Milky Method making an attempt to develop into a disk galaxy,” says Belokurov, who noticed the identical spin-up in analysis that he and a colleague reported in July.
As we speak, the Milky Method is a huge galaxy that spins quickly — every hour our photo voltaic system speeds via 900,000 kilometers of area as we race across the galaxy’s heart. However the brand new examine reveals that the Milky Method acquired its begin as a modest protogalaxy whose stars nonetheless shine immediately, stars that astronomers can now scrutinize for additional clues to the galaxy’s delivery and early evolution.
