Large dark-matter halo discovered in ancient dwarf galaxy

February 4, 2021.
The vicinity of the Tucana II ultra-faint dwarf galaxy, as imaged with the SkyMapper Telescope. (Anirudh Chiti, MIT)

The vicinity of the Tucana II ultra-faint dwarf galaxy, as imaged with the SkyMapper Telescope. (Anirudh Chiti, MIT)

Stars discovered on the outskirts of a small nearby galaxy have led astronomers to calculate that it contains a much larger disc of dark matter than previously thought, and the findings provide new clues to understanding how the universe’s first galaxies formed.

A paper published Monday in Nature Astronomy detailed fresh observations of Tucana II, a faint group of about 3,000 stars orbiting the Milky Way Galaxy and one of the oldest known galaxies. Researchers from the U.S., U.K. and Australia found stars in Tucana II that were far from its center, a sign of abundant unseen matter and an ancient supernova or galactic collision.

Tucana II is about 163,000 light years from Earth and is one of many dwarf galaxies revolving around the Milky Way that were formed shortly after the Big Bang. At more than 13 billion years old, it provides a rare look into early star and galaxy formation, said Anirudh Chiti, a graduate student at the Massachusetts Institute of Technology and the study’s lead author. 

The dim, faraway stars in Tucana II contain very small amounts of elements heavier than hydrogen and helium due to their old age. Chiti developed an imaging technique that made these “metal-poor” stars stand out in the night sky during observations from Australia’s SkyMapper telescope. 

“They were literally blinking in our face all around the galaxy,” said Anna Frebel, an MIT professor and the senior author of the paper. “It was a magical moment.”

Some of the nine stars the researchers discovered were more than three times farther from the galaxy’s center than its 10 previously identified stars. They concluded the far-out stars could remain in Tucana II only if the galaxy had an extended halo of dark matter, a ubiquitous yet mysterious form of matter that can be detected only through its gravitational effects on stars and other visible matter.

“We’ve always known that these galaxies probably have a lot of dark matter just in their central regions,” Chiti said. “But what this tells us is that there's actually a distribution of dark matter that extends out to very large distances from the center of these galaxies.”

Tucana II’s dark-matter halo is the first direct evidence to support early-universe simulations predicting the first galaxies would have lots of dark matter, according to Frebel, an expert in early-universe stars. It may also indicate that other ultra-faint galaxies have extended dark-matter halos that have not been previously observed.

"The luminous mass of the stars and the gas need to be held together," Frebel said, "and simulations have shown that you need that much dark matter to even be able to create such a cradle, otherwise stuff will just fall apart."

Tucana II’s wide distribution of dark matter suggests it underwent an early disruptive event such as a supernova or galaxy merger, which would be among the first to occur in the universe, according to the researchers. The dwarf galaxy’s evolution could help illuminate the history of the Milky Way and other galaxies, which were likely formed long ago from small clusters of stars similar to Tucana II.

The researchers’ observations also confirmed Tucana II is the most metal-poor known galaxy, with the newly discovered stars driving its average metallicity even lower.

Chiti and Frebel hope to confirm the broader implication of their findings by applying the same imaging technique to the other ancient galaxies orbiting the Milky Way. 

“We can basically just set up the infrastructure to do this on pretty much nearly every primitive galaxy that's orbiting the Milky Way,” Chiti said. “We’ll manage to have a comprehensive look at the dark-matter contents and maybe the prevalence of mergers in the early universe.”

The article, "An extended halo around an ancient dwarf galaxy," was published Feb. 1 in Nature AstronomyThe authors of the study were Anirudh Chiti and Anna Frebel, Massachusetts Institute of Technology; Joshua Simon, Lina Necib and Alexander Ji, Observatories of the Carnegie Institution for Science; Denis Erkal, University of Surrey; Laura Chang, Princeton University; Helmut Jerjen and John Norris, Australian National University; and Dongwon Kim, University of California, Berkeley. The lead author was Anirudh Chiti.

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