Extended dark matter halo found circling ancient dwarf galaxy

Feb. 1 (UPI) -- Astronomers have discovered an extended dark matter halo encircling Tucana II, an ultra faint, ancient dwarf galaxy located 163,000 light years from Earth.

The discovery, described Monday in the journal Nature Astronomy, suggests the cosmos' first galaxies were more massive than scientists thought.

Tucana II is one of dozens of tiny galaxies caught in the orbit of the Milky Way. Astronomers have long suspected that Tucana II and its peers are remnants of the first generation of galaxies.

While surveying Tucana II, scientists observed a spattering of distant stars, far from the miniature galaxy's center but under the influence of the galaxy's gravitational pull.

"Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away," lead study author Anirudh Chiti, MIT graduate student, said in a news release. "This means that other relic first galaxies probably have these kinds of extended halos too."

Researchers determined Tucana II's distant stars are older than those within the galaxy's core. The dichotomy may be evidence of an ancient galactic merger.

"We may be seeing the first signature of galactic cannibalism," said co-author Anna Frebel, an associate professor of physics at MIT. "One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts."

Because Tucana II features very low metal concentrations, scientists have long assumed the dwarf galaxy first formed several billion years ago. Early in the history of the universe, heavy metals were rare.

Earlier surveys of Tucana II revealed metal-poor stars in its center, but astronomers wondered whether the galaxy's outskirts might host even more ancient stars with even smaller concentrations of metal.

For the latest survey, scientists applied an imaging filter to a ground-based telescope in Australia, the SkyMapper Telescope. An algorithm developed by Chiti helped the team process the filtered data and isolate the most metal-poor stars.

The analysis revealed a link between the low-metal stars in the galaxy's center and outskirts.

"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.

Researchers estimate the inner and outer stars are linked by an invisible web of dark matter. It's the first time scientists have identified a dark matter halo surrounding an ultrafaint dwarf galaxy.

"Without dark matter, galaxies would just fly apart," Chiti said. "[Dark matter] is a crucial ingredient in making a galaxy and holding it together."

The findings suggest many more of the cosmos' earliest galaxies may have formed with in expansive dark matter halos.

After identifying the links between the inner and out stars, researchers observed the galaxy using Chile's Magellan Telescopes.

The images confirmed the dwarf galaxy's outer stars are more metal-poor than the stars in its core, evidence that Tucana II is the product of an ancient episode of cannibalism.

Just as Tucana II once grew by consuming its neighbor, Tucana II itself will one day be swallowed up by the Milky Way.

Researchers plan to use the same image-filtering techniques that revealed Tucana II's distant stellar population to locate ancient stars orbiting other dwarf galaxies.

"There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts," Frebel said.