Astronomers discover halo of dark matter surrounding ancient dwarf galaxy

Invisible, elusive dark matter is believed to make up about 85 per cent of all the matter in the universe

A team of astronomers from MIT claim to have found evidence of a vast halo of dark matter halo around an ancient galaxy.

The team say they have identified nine stars at the edge of the galaxy Tucana II. Thesstars are located a long way from the galaxy's centre, but are still caught in its gravitational pull, suggesting that Tucana II hosts an extended dark matter halo (a region of gravitationally bound matter), which is invisible to the eye but exerts gravitational force on the stars themselves.

Calculations now suggest that the dark matter halo around is about three to five times larger than previously thought.

The concept of dark matter has perplexed scientists for decades. The invisible, elusive material is believed to make up about 85 per cent of all the matter in the universe, and to account for about 25 per cent of the universe's total energy density. Its has never been observed directly, and scientists predict its presence only on the basis of its gravitational effect on other objects in the universe.

"Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away," said MIT graduate student Anirudh Chiti.

"This means that other relic first galaxies probably have these kinds of extended halos too."

Tucana II galaxy is an ancient, ultrafaint dwarf galaxy, about 163,000 light-years from Earth. According to scientists, our Milky Way galaxy is surrounded by dozens of such dwarf satellite galaxies, which are the remnants of the earliest galaxies.

To identify new stars in Tucana II, the researchers used images captured by the Australian National University SkyMapper telescope, along with data from the Gaia satellite.

They discovered that the newly identified stars at the edge of Tucana II are up to about 3,500 light years from the centre of the galaxy, compared with up to about 1,100 light years for previously known stars. The newly found stars were also found to contain lower metal content, an indication that they are much older.

The extended nature of the galaxy suggests that stars on the edge are older than those at its core. That stellar imbalance could have been the result of an early galactic merger, where two infant galaxies collided with each other and merged to create a new galaxy.

Astronomers believe that Tucana II will eventually be eaten by own Milky Way.

The detailed findings of the study are published in the journal Nature Astronomy.