Astronomers Detect Extended Dark Matter Halo Around Ancient Dwarf Galaxy

The vicinity of the Tucana II ultra-faint dwarf galaxy, as imaged with the SkyMapper Telescope Credits:Image: Anirudh Chiti, MIT

The Milky Way is surrounded by dozens of dwarf galaxies that are thought to be relics of the very first galaxies in the universe. Among the most primitive of these galactic fossils is Tucana II—an ultrafaint dwarf galaxy that is about 50 kiloparsecs, or 163,000 light years, from Earth.

Now MIT astrophysicists have detected stars at the edge of Tucana II, in a configuration that is surprisingly far from its center but nevertheless caught up in the tiny galaxy’s gravitational pull. This is the first evidence that Tucana II hosts an extended —a region of gravitationally bound matter that the researchers calculated to be three to five times more massive than scientists had estimated. This discovery of far-flung stars in an ancient dwarf galaxy implies that the very first  in the universe were also likely extended and more massive than previously thought.

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

The researchers also determined that the stars on the outskirts of Tucana II are more primitive than the stars at the galaxy’s core. This is the first evidence of such a stellar imbalance in an ultrafaint dwarf galaxy.

The unique configuration suggests that the ancient galaxy may have been the product of one of the first mergers in the universe, between two infant galaxies—one slightly less primitive than the other.

“We may be seeing the first signature of galactic cannibalism,” says Anna Frebel, the Silverman Family Career Development 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.”

Frebel, Chiti, and their colleagues have published their results today in Nature Astronomy.

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SOURCE:, Jennifer Chu

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