Astronomers have found an ancient galaxy with a halo of dark matter

Some 163000 lightyears of the Milky Way is a much smaller and much older galaxy: Tucana II, so named because of the tropical bird-like constellation it is in. Sitting on the outskirts of our galaxy’s gravitational pull, Tucana II offers researchers the opportunity to understand the makeup of the universe’s earliest galactic structures.

Now a team of astronomers has found evidence of a dark matter halo extended around the galaxy. Their research was published today in the journal Nature Astronomy.

“We know [dark matter] is there because for galaxies to stay linked there has to be more matter than what we can visibly see in starlight, ”said Anirudh Chiti, astronomer at the Massachusetts Institute of Technology, during a phone call. “This led to the hypothesis of a dark matter existing as an ingredient that holds galaxies together; without it, the galaxies we know, or at least those on their periphery, would only be shattered.

A dark matter halo is a gravitational region matter bound in space. (The dark matter halo of the Milky Way extends far beyond the pinwheel which constitutes the visible substance of our galaxy). The team found that the gravitational limits of Tucana II are between three and five times more massive than previously thought, showing that even some of the oldest galaxies will have dark matter halos.

Tucana II happens to be the most chemically primitive galaxy we know of today, which means that some of its stars have very low metal content (the heaviest elements in the universe were produced later in time). The team realized that Tucana II had the dark matter halo when star observations in this region of the sky revealed that the stars were moving in tandem.

“If you just look at the region of the sky where the galaxy is located, you don’t actually see a clustering or overdensity of stars,” Chiti, who is main author of the recent article, said. “It’s only when you look at their speeds and realize that it’s a group of stars moving at the same speed that you realize that there is a galaxy that exists there.”

As study co-author Anna Frebel, also an astronomer at MIT, wrote in an academic press release, the Tucana II spiral of movement looks like “bathwater going down the drain.” Undoubtedly, some of the peripheral stars in the galaxy are older than the stars closer to the galactic center. The team speculates that Tucana II could be the result of a previous galactic merger, a cosmic clash that saw one primitive galaxy consumed by another, resulting in stars of different origins in the same galaxy.

Whether this theory of the origin of Tucana II is true or not, a similar collision is certainly in its future.. Since it is in the gravitational realm of the much more massive Milky Way, the relatively small galaxy will eventually be engulfed by ours.

While astronomers know how to spot dark matter halos, they still don’t know exactly what dark matter is. In addition to finding its halos around galaxies, researchers are also looking for the identity of dark matter in mysterious neutron star signals and in the form of tiny theoretical black holes.