Astronomers using with the Australian National University 1.3-m SkyMapper telescope have detected an extended dark matter halo around Tucana II, an ultrafaint dwarf galaxy located 163,000 light-years away from Earth. The discovery, reported in the journal Nature Astronomy, implies that the very first galaxies in the Universe were also likely extended and more massive than previously thought.
This image from the SkyMapper Telescope shows the vicinity of the Tucana II galaxy. Image credit: Anirudh Chiti, MIT.
The Milky Way is surrounded by dozens of ultrafaint dwarf satellite galaxies. They are the remnants of very ancient galaxies, as confirmed by their old and chemically primitive stars.
Tucana II, a typical ultrafaint dwarf galaxy, is extremely dark matter dominated, has a low stellar mass and a low metallicity.
Metal-poor stars likely formed very early on, when the Universe was not yet producing heavy elements.
In the case of Tucana II, astronomers had previously identified a handful of stars around the galaxy’s core with such low metal content that the galaxy was deemed the most chemically primitive of the known ultrafaint dwarf galaxies.
Anirudh Chiti, a graduate student in the Department of Physics and the Kavli Institute for Astrophysics and Space Research at MIT, and colleagues wondered whether Tucana II might harbor other, even older stars, that might shed light on the formation of the Universe’s first galaxies.
They used an imaging filter on the SkyMapper Telescope to spot primitive, metal-poor stars beyond the galaxy’s core.
They then ran an algorithm through the filtered data to efficiently pick out stars with low metal content.
Such stars were detected out to about nine times the half-light radius of Tucana II, far beyond member stars in other ultrafaint dwarf galaxies.
“Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away,” Chiti said.
“This means that other relic first galaxies probably have these kinds of extended halos too.”
The results suggest that Tucana II must have an extended dark matter halo that is 3-5 times more massive than previously thought, in order for it to keep a gravitational hold on these far-off stars.
“This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought,” said Dr. Anna Frebel, also form the Department of Physics and the Kavli Institute for Astrophysics and Space Research at MIT.
“We have thought that the first galaxies were the tiniest, wimpiest galaxies. But they actually may have been several times larger than we thought, and not so tiny after all.”
Using data from the Magellan telescopes, the astronomers discovered Tucana II’s outer stars were three times more metal-poor, and therefore more primitive, than those at the center.
“This is the first time we’ve seen something that looks like a chemical difference between the inner and outer stars in an ancient galaxy,” Chiti said.
A likely explanation for the imbalance may be an early galactic merger, in which a small galaxy — possibly among the first generation of galaxies to form in the Universe — swallowed another nearby galaxy.
The team plans to use their approach to observe other ultrafaint dwarf galaxies around the Milky Way, in hopes of discovering even older, farther-flung stars.
“There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts,” Dr. Frebel said.
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A. Chiti et al. An extended halo around an ancient dwarf galaxy. Nat Astron, published online February 1, 2021; doi: 10.1038/s41550-020-01285-w
