A protogalaxy in the Milky Way may be our galaxy’s original nucleus

The Milky Approach left its “poor outdated coronary heart” in and across the constellation Sagittarius, astronomers report.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 authentic nucleus — the traditional stellar inhabitants that the remainder of the Milky Approach grew round — which got here collectively greater than 12.5 billion years in the past.

“Individuals have lengthy speculated that such an enormous inhabitants [of old stars] ought to exist within the heart of our Milky Approach, and Gaia now exhibits that there they’re,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.

The Milky Approach’s historical coronary heart is a spherical protogalaxy that spans practically 18,000 light-years and possesses roughly 100 million times the mass of the sun in stars, or about 0.2 p.c of the Milky Approach’s present stellar mass, Rix and colleagues report in a research posted September 7 at arXiv.org.

“This research actually helps to agency up our understanding of this very, very, very younger stage within the Milky Approach’s life,” says Vasily Belokurov, an astronomer on the College of Cambridge who was not concerned within the work. “Not a lot is de facto identified about this era of the Milky Approach’s life,” he says. “We’ve seen glimpses of this inhabitants earlier than,” however the brand new research offers “a chicken’s-eye view of the entire construction.”

Most stars within the Milky Approach’s central area abound with metals, as a result of the celebs originated in a crowded metropolis that earlier stellar generations had enriched with these metals by 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 crew turned to knowledge from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way (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, in search of stars with metal-to-hydrogen ratios not more than 3 p.c of the solar’s.

The astronomers then examined how these stars transfer by house, retaining solely those that don’t dart off into the huge halo of metal-poor stars engulfing the Milky Approach’s disk. The top consequence: a pattern of 18,000 historical stars that represents the kernel round which your 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 Approach’s oldest stars, which he says most likely have ages exceeding 12.5 billion years. The protogalaxy fashioned when a number of massive clumps of stars and gasoline conglomerated way back, earlier than the Milky Approach’s first disk — the so-called thick disk — arose (SN: 3/23/22).

The protogalaxy is compact, which suggests little has disturbed it since its formation. Smaller galaxies have crashed into the Milky Approach, 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 Approach’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 Approach barely revolve across the galaxy’s heart however dive out and in of it as a substitute, whereas barely youthful stars present an increasing number of motion across the galactic heart. “That is the Milky Approach making an attempt to change into a disk galaxy,” says Belokurov, who noticed the same spin-up in analysis that he and a colleague reported in July.

At present, the Milky Approach is a big galaxy that spins quickly — every hour our photo voltaic system speeds by 900,000 kilometers of house as we race across the galaxy’s heart. However the brand new research exhibits that the Milky Approach obtained its begin as a modest protogalaxy whose stars nonetheless shine right this moment, stars that astronomers can now scrutinize for additional clues to the galaxy’s beginning and early evolution.

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