Just as the Moon orbits the Earth, our galaxy, the Milky Way, has many orbiting satellite galaxies. These small galaxies are called dwarf galaxies and are thought to be remnants of galaxies that formed at the beginning of the universe.
One of the dwarf galaxies around the Milky Way is called Tucana II, an ultra-faint dwarf galaxy about 163,000 light-years from Earth. To the surprise of astronomers, despite its small size, Tucana II can be seen gravitationally from a great distance from its center, suggesting that the dwarf galaxy has a lot of dark matter inside.
Based on their analysis, astronomers found that the visible stellar mass of Tucana II is about 3,000 times the mass of the Sun, but its dark matter halo is about 10 million times the mass of the Sun. This is three to five times larger than previous estimates.
“Tucana II is much more massive than we thought to gravitationally bind such a distant star,” Anirudh Chiti, an astrophysicist at the Massachusetts Institute of Technology, said in a statement. “This means that other early galaxies may also have these types of dark matter halos.”
The new findings suggest that early galaxies in the universe may have been much more massive than previously thought.
Astronomers often use metal abundances to estimate the age of galaxies. Because metals take a long Time to form in stars and then propagate throughout the universe, very early galaxies are unable to accumulate large amounts of metals.
Based on the metal abundance of Tucana II, it is one of the oldest galaxies, and Chiti and his team are studying the stars in Tucana II in hopes of finding older stars.
To their surprise, they found nine new stars that are far from the center of Tucana II.
Data collected by the European Space Agency’s Gaia Telescope confirmed their results. Those stars far from the cores of dwarf galaxies are gravitationally bound.
However, previous estimates of the mass of Tucana II are insufficient to produce such a strong gravitational force. This implies the presence of a large amount of mass that we cannot see directly, i.e., dark matter.
So far, it is not clear what dark matter is, but there is much observational evidence, especially through the gravitational pull of galaxies, pointing to the presence of undetectable mass. In fact, cosmologists believe that about 85 percent of the matter in the universe is dark matter.
“Without dark matter, galaxies would fly apart,” Chitty said. “Dark matter is the key element that makes galaxies and keeps them together.”
Based on the location and motion of the stars, the team was able to estimate the amount of dark matter in Tucana II, eventually reaching the 10 million solar mass range. This is the first evidence that a superfaint dwarf galaxy like Tucana II could have that much dark matter.
“This may also mean that the earliest galaxies formed with much larger dark matter halos than previously thought,” says MIT astrophysicist Anna Frebel. “We thought that early galaxies were some of the smallest galaxies. But they may actually be several times larger than we thought, and not quite as small.”
But there are more questions about this observation. When the team analyzed data from the Magellan Telescopes, they found that the metal abundances of the stars in Tucana II may differ.
In fact, the stars are divided into two groups. the metal abundances of the stars in the outer part of Tucana II are three times lower than those in the core.
In general, this happens if a large number of stars may come from elsewhere (e.g. galaxy mergers).
Such chemical differences between stars have never been seen in ancient galaxies. This leads to a very interesting theory: In the early days, Tucana II was not one, but two galaxies that merged together and their dark matter halos also merged.
“We may be seeing the first features of galaxy mergers,” Freibel said. “One galaxy may have swallowed up one of its slightly smaller, more primitive neighbors and then squeezed all its stars to the outside.”
The team plans to expand its technique to include more stars and galaxies.
Freibel said, “There are probably more systems, perhaps all systems, where these stars are shimmering on their outer sides.”
This new work has been published in the Feb. 1, 2021, issue of Nature Astronomy.
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