Dwarf galaxy Tucana II has an extended dark matter halo

Our galaxy is surrounded by dozens of dwarf galaxies that scientists believe are remnants of the very first galaxies in the universe. One of the oldest of those galaxies is an ultra-faint dwarf galaxy about 163,000 light-years from Earth called Tucana II. Astrophysicists at MIT have now detected stars at the edge of Tucana II that are in a configuration surprisingly far from the center of the galaxy but are caught in the galaxy's gravitational pull.Researchers believe this is the first sign that Tucana II has an extended dark matter halo. The halo is a region of gravitationally bound matter researchers calculate to be 3 to 5 times more massive than previously estimated. This discovery implies that the first galaxies in the universe were also likely extended and more massive than previously believed.

Researchers have also determined that stars on the outside of Tucana II are more primitive than the stars at the galaxy's core. They believe this is the first evidence of this type of stellar imbalance in an ultra-faint dwarf galaxy. The unique configuration suggests the ancient galaxy could've been the product of one of the first mergers in the universe between a pair of infant galaxies with one slightly less primitive than the other.

Researcher Anna Frebel says that this could be the first sign of "galactic cannibalism." She believes that one galaxy may have eaten a slightly smaller and more primitive neighbor, spilling stars into the outskirts of the combined galaxy. Tucana II is one of the most primitive dwarf galaxies known to exist, a statement based on the metal content of its stars.

Researchers believe that stars with low metal content formed very early when the universe wasn't producing heavy elements. In the case of Tucana II, a handful of stars around the galactic core had such low metal content the galaxy was deemed the most chemically primitive of all known ultra-faint dwarf galaxies.