Galactic Cannibalism May Explain Newly Detected Dark Matter Halo

Scientists found that one of the Milky Way’s galactic neighbors is much more massive than previously believed, implying the existence of an extended halo of dark matter that could be the result of one ancient galaxy swallowing up another.

Four antennas used in a star-gazing project point up at the night sky in Chile’s Atacama Desert. (ESO Photo Ambassador José Francisco Salgado)

(CN) — Astrophysicists say that an extended halo of dark matter may surround Tucana II, one of the many dwarf galaxies neighboring the Milky Way. The ancient galaxy may also have been the site of one of the universe’s earliest galactic collisions.

A bona fide celestial fossil, all signs point to Tucana II’s extremely old age. The scientists say Tucana II, located a cozy 163,000 light years from Earth, is “the most metal-poor known galaxy” — suggesting the galaxy was formed before the universe was producing heavy elements. Astronomers call these ultra-faint dwarf galaxies.

These are the galaxies richest in dark matter, that difficult-to-detect substance that apparently interacts with no form of observable electromagnetic radiation — such as visible light, radio waves or gamma rays — but whose existence is implied by gravitational effects that could not otherwise be explained.

Dark matter could explain, for instance, why galaxies don’t fly apart. Scientists rely on extremely sensitive imaging equipment, like high-resolution cameras and massive optical telescopes, to be able to infer dark matter’s presence in neighboring galaxies.

The concept of dark matter also helps answer questions such as why the stars at the edge of Tucana II still circle the galaxy’s center when all the observable matter suggests that the small galaxy should exert no pull on them.

“Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away,” said Anirudh Chiti, study co-author and Massachusetts Institute of Technology graduate student, in a statement. “This means that other relic first galaxies probably have these kinds of extended halos too.”

Chiti works with Anna Frebel, astrophysicist and principal investigator for the MIT Kavli Institute for Astrophysics and Space Research. With the help of observations taken with the SkyMapper Telescope, a wide-field survey telescope in southeastern Australia, Chiti and Frebel pinpointed stars poor in metals located outside of the galaxy’s core.

Chiti developed an algorithm that picked out metal-poor stars, identifying nine new ones far from Tucana II’s galactic core. 

“Ani’s analysis shows a kinematic connection, that these far-out stars move in lockstep with the inner stars, like bathwater going down the drain,” Frebel said in the statement. Their findings, co-produced with six other scientists, were published Monday in the scientific journal Nature Astronomy.

How does the galaxy maintain a gravitational pull on those faraway stars? An extended halo of dark matter, between three and five times more massive than first estimated, would explain the unusual extension of Tucana II’s most far-flung stars.

“Without dark matter, galaxies would just fly apart,” Chiti said, adding that dark matter “is a crucial ingredient in making a galaxy and holding it together.”

Though it is not yet possible for scientists to directly observe dark matter halos, halos have been inferred close to home: our own Milky Way galaxy is host to the unobservable matter. Monday’s research is the first evidence of an ultra-faint dwarf galaxy accommodating an extended halo of dark matter.

“This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought,” Frebel said. “We have thought that the first galaxies were the tiniest, wimpiest galaxies. But they actually may have been several times larger than we thought, and not so tiny after all.”

With the help of Chile’s Magellan Telescopes, a pair of optical telescopes each measuring 21 feet across, the researchers found that Tucana II’s outermost stars were three times as metal-poor than the stars at the center of the galaxy.

“This is the first time we’ve seen something that looks like a chemical difference between the inner and outer stars in an ancient galaxy,” Chiti said.

The group proposes that this difference could be explained by an early galactic merger.

“We may be seeing the first signature of galactic cannibalism,” Frebel said. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts.”

Galactic mergers are commonplace in the modern universe.

“Tucana II will eventually be eaten by the Milky Way, no mercy,” Frebel added. “And it turns out this ancient galaxy may have its own cannibalistic history.”

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