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A Protogalaxy in the Milky Way May Be Our Galaxy’s Original Nucleus

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A Protogalaxy in the Milky Way May Be Our Galaxy’s Original Nucleus. Credit: Terence Dickinson / ESA

Astronomers have identified a protogalaxy in the Milky Way in and around the constellation Sagittarius which suggests it may be our galaxy’s original nucleus.

New data from the Gaia spacecraft reveal the full extent of what seems to be the galaxy’s original nucleus—the ancient stellar population that the rest of the Milky Way developed around—which came together over 12.5 billion years ago.

“People have long speculated that such a vast population [of old stars] should exist in the center of our Milky Way, and Gaia now shows that there they are,” says astronomer Hans-Walter Rix of the Max Planck Institute for Astronomy in Heidelberg, Germany.

In a study posted on September 7th, Rix and colleagues reported that the Milky Way’s ancient heart is a round protogalaxy that spans nearly eighteen thousand light-years and possesses roughly one hundred million times the mass of the sun in stars, or about 0.2 percent of the Milky Way’s current stellar mass.

“This study really helps to firm up our understanding of this very, very, very young stage in the Milky Way’s life,” 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 gives “a bird’s-eye view of the whole structure,” he added.

Stars in the Milky Way Bound With Metals

Most stars in the protogalaxy in the Milky Way’s central region are bound with metals because the stars originated in a crowded capital that earlier stellar generations had enriched with those metals through supernova explosions.

Rix and his colleagues wanted to find the exceptions to the rule, stars so metal-poor they must have been born well before the rest of the galaxy’s stellar denizens came along—what Rix calls “a needle-in-a-haystack exercise.”

His team turned to data from the Gaia spacecraft, which launched in 2013 on a mission to chart the Milky Way (SN: 6/13/22).

The astronomers searched about two million stars within a broad region around the galaxy’s center, which lies in the Sagittarius constellation, looking for stars with metal-to-hydrogen ratios no more than three percent of that of the sun.

The astronomers then examined how those stars move through space, retaining only the ones that don’t dart off into the vast halo of metal-poor stars.

Protogalaxy Close to 200 Million Times as Massive as Sun

According to an end result of astronomers’ findings, a sample of eighteen thousand ancient stars represents the kernel around which the entire galaxy blossomed, researchers say. By accounting for stars obscured by dust, Rix estimates that the protogalaxy is between fifty million and two hundred million times as massive as the sun.

“That’s the original core,” Rix says, and it harbors the Milky Way’s oldest stars, which he says probably have ages exceeding 12.5 billion years.

The protogalaxy formed when several large clumps of stars and gas conglomerated long ago before the Milky Way’s first disk—the so-called thick disk—arose. This implies that the protogalaxy is compact, meaning little has disturbed it since its formation.

Rix says smaller galaxies have crashed into the Milky Way, augmenting its mass, but “we didn’t have any later mergers that deeply penetrated into the core and shook it up…because then the core would be larger now.”

The new data on the protogalaxy even capture the Milky Way’s initial spin-up—its transition from an object that didn’t rotate into one that now does.

Protogalaxy Milky Way Oldest Stars Just Revolve Around Center of Galaxy

The oldest stars in the proto–Milky Way barely revolve around the galaxy’s center but dive in and out of it instead whereas slightly younger stars show more and more movement around the galactic center.

Belokurov, who saw the same spin-up in research that he and a colleague reported in July says, “This is the Milky Way trying to become a disk galaxy.”

Today, the Milky Way is a giant galaxy that spins rapidly. Each hour our solar system speeds through nine hundred thousand kilometers of space as we race around the galaxy’s center.

However, a new study shows that the protogalaxy in the Milky Way got its start as a modest protogalaxy whose stars still shine today—stars that astronomers can now scrutinize for further clues to the galaxy’s birth and early evolution.

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