Numerous dwarf galaxies that are believed to be remnants of the universe’s very first galaxies encircle the Milky Way. Tucana II, an ultrafaint dwarf galaxy that is around 50 kiloparsecs, or 163,000 light years, from Earth, is one of the most ancient of these galactic relics.
In a configuration that is unexpectedly far from Tucana II’s center but is yet drawn in by its gravitational pull, MIT astronomers have now found stars at the galaxy’s periphery.
This is the first proof that Tucana II contains a long dark matter halo, an area of gravitationally bound matter that the researchers predicted to be three to five times more massive than previous estimates.
This finding of distant stars in an old dwarf galaxy suggests that the early galaxies in the universe were probably much more massive and stretched than previously imagined.
“Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away,” says MIT graduate student Anirudh Chiti. “This means that other relic first galaxies probably have these kinds of extended halos too.”
The scientists also discovered that Tucana II’s outer stars are more primitive than the galaxy’s central stars. This is the first instance of such a star imbalance in a dwarf galaxy that is extremely dim.
The ancient galaxy may have been created by one of the universe’s first mergers of two young galaxies, one of which was somewhat less primitive than the other, according to its unusual structure.
“We may be seeing the first signature of galactic cannibalism,” says Anna Frebel, the Silverman Family Career Development Associate Professor of Physics at MIT. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts.”
Frebel, Chiti, and their colleagues have published their results today in Nature Astronomy.
Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away. This means that other relic first galaxies probably have these kinds of extended halos too.
Anirudh Chiti
Not-so-wimpy galaxies
Based on the amount of metal in its stars, Tucana II is one of the oldest known dwarf galaxies. Low metal content stars probably created extremely early, before the cosmos began to produce heavy elements.
Tucana II was the most chemically basic of the known ultrafaint dwarf galaxies since astronomers had previously discovered a few stars near the galaxy’s core with a very low metal content.
Chiti and Frebel pondered the possibility that the old galaxy might include more, even older stars that could provide information on how the universe’s first galaxies came into being. They gathered observations of Tucana II using the SkyMapper Telescope, an optical ground-based telescope in Australia that captures expansive views of the southern sky, in order to put their theory to the test.
To find primitive, metal-poor stars outside of the galaxy’s core, the scientists used an image filter on the telescope. The scientists used an algorithm created by Chiti to efficiently identify low metal content stars, including the previously reported stars at the galactic center and nine new stars that were located far farther from the galactic core.
“Ani’s analysis shows a kinematic conection, that these far-out stars move in lockstep with the inner stars, like bathwater going down the drain,” Frebel adds.
The findings imply that for Tucana II to maintain a gravitational hold on these far-off stars, it must have an extended dark matter halo that is three to five times more massive than previously assumed.
More than 85% of the universe is assumed to be made up of dark matter, a hypothetical kind of stuff. Every galaxy is believed to be held together by a small concentration of dark matter, known as a halo.
“Without dark matter, galaxies would just fly apart,” Chiti. says. “Dark matter is a crucial ingredient in making a galaxy and holding it together.”
The team’s findings provide the first concrete proof that a dim dwarf galaxy may support a substantial dark matter halo.
“This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought,” Frebel says. “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.”
“A cannibalistic history”
Following up on their first findings, Chiti and Frebel observed Tucana II using the Chilean Magellan Telescopes. The scientists utilized Magellan to narrow in on the galaxy’s metal-poor stars and determine their relative metallicities. They found that the outer stars were three times more metal-poor than the central stars, making them more primordial.
“This is the first time we’ve seen something that looks like a chemical difference beween the inner and outer stars in an ancient galaxy,” Chiti says.
An early galactic merger, in which a small galaxy probably among the first generation of galaxies to originate in the cosmos absorbed another neighboring galaxy, may be a possible explanation for the imbalance. Despite the fact that the cosmos is currently experiencing ongoing galactic cannibalism, it was previously unknown whether early galaxies had fused in a similar manner.
“Tucana II will eventually be eaten by the Milky Way, no mercy,” Frebel says. “And it turns out this ancient galaxy may have its own cannibalistic history.”
In order to find even older, more distant stars, the team intends to utilize their method to observe other ultradim dwarf galaxies in the Milky Way’s neighborhood.
“There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts,” Frebel says.
This research was supported in part by NASA and the National Science Foundation.
