TORONTO -- A team of astronomers from Massachusetts Institute of Technology (MIT) have found evidence of a vast halo of dark matter around an ancient dwarf galaxy, suggesting the first galaxies in the universe were larger than previously thought.
Researchers say they have identified a configuration of nine stars at the edge of an "ultrafaint dwarf galaxy," known as Tucana II, approximately 163,000 light years from Earth. According to the study, the stars showed to be caught in Tucana II’s gravitational pull despite being far from the galaxy's centre.
The study, , noted that the extended dark matter halo is a "region of gravitationally bound matter" that is invisible to the eye, but exerts gravitational force on the stars themselves.
Researchers say this is the first evidence that the Tucana II galaxy hosts an extended dark matter halo calculated to be "three to five times more massive" than scientists had previously estimated.
The study reported that dark matter is a "hypothetical type of matter" that is thought to make up more than 85 per cent of the universe. Every galaxy is thought to be held together by a local concentration, or halo, of dark matter, the study says.
"Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away," MIT graduate student Anirudh Chiti said in a . "This means that other relic first galaxies probably have these kinds of extended halos too."
According to scientists, the Milky Way galaxy is surrounded by dozens of dwarf galaxies, which are the remnants of the earliest galaxies.
The researchers also found that these stars on the outskirts of Tucana II were more primitive than the stars at the galaxy's core, suggesting a "stellar imbalance in an ultrafaint dwarf galaxy" due to their metal content.
'GALACTIC CANNIBALISM'
According to the study, the new stars were located up to 3,500 light years from the centre of the galaxy, compared to approximately 1,100 light years for the previously known stars.
The study, which was supported in part by NASA and the National Science Foundation, reported that Tucana II's "unique configuration" of stars suggests the ancient galaxy may have formed from a merging of two younger galaxies.
Associate professor of physics at MIT Anna Frebel said this may be the "first signature of galactic cannibalism" in the universe.
"One galaxy may have eaten one of its slightly smaller, more primitive neighbours, that then spilled all its stars into the outskirts," Frebel explained in the release.
According to the study, this "galactic cannibalism" of one galaxy swallowing or colliding with another occurs constantly throughout the universe today, but it was unclear to astronomers whether early galaxies merged in a similar way.
"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," Frebel said.
Frebel noted that future merging will also likely lead to the end of Tucana II.
"Tucana II will eventually be eaten by the Milky Way, no mercy. And it turns out this ancient galaxy may have its own cannibalistic history," Frebel said.
SEARCHING FOR NEW STARS
To identify new stars in Tucana II, the researchers used images captured by the Australian National University SkyMapper telescope as well as data from the Gaia satellite. The initial results were followed up with observations of Tucana II taken by the Magellan Telescopes in Chile.
Researchers used an imaging filter on the SkyMapper telescope to spot "primitive, metal-poor stars" beyond Tucana II’s core. Using an algorithm researchers were able to filter out stars with low metal content, including the previously identified stars at the galaxy's centre and the nine new stars further from the galactic core.
"[The] analysis shows a kinematic connection, that these far-out stars move in lockstep with the inner stars, like bathwater going down the drain," Frebel said in the release.
Using the Magellan Telescopes, the researchers were able to focus closer on the metal content of these stars. Chiti said in the release that researchers found the outer stars were three times more metal-poor, and therefore "more primitive" or older than those at Tucana II's core.
"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.
The findings suggest that a much larger dark matter halo than initially thought would have to surround Tucana II in order for it to keep a gravitational hold on these far-off stars.
"Without dark matter, galaxies would just fly apart," Chiti said in the release. "[Dark matter] is a crucial ingredient in making a galaxy and holding it together."
Researchers say they plan to use the methods in this study to observe more ultrafaint dwarf systems around the Milky Way with the hope of discovering "even older, farther-flung stars" that are likely on the outskirts of other galaxies.
