Two mysterious galaxies devoid of dark matter could have a striking history of origin.
Researchers suggest that about 8 billion years ago, two dwarf galaxies collided. This cosmic collapse caused the gas in these two galaxies to split and form many new dwarf galaxies, including two without dark matter.
Newly discovered row of dwarf galaxiesmore than 6 million light-years long, it may have formed after the alleged crash, researchers said in a May 19 publication. nature. If true, the discovery could help solve the mystery of how such unusual galaxies form without dark matter and reveal new details about the nature of dark matter.
But other scholars are skeptical that there is enough evidence to support this prehistory. “If that’s true, I think it would be really exciting. I just don’t think the bar has been met, “said astronomer Michelle Collins of Surrey University in Guildford, England.
In 2018, Yale University astronomer Peter van Docum and colleagues reported a dwarf galaxy without dark matter (SN: 28.03.18). Invisible, mysterious matter is usually found in galaxies through its gravitational effects on stars. When a second dwarf galaxy without dark matter was discovered in the vicinity in 2019, this raised an obvious question: How did the two strange galaxies form? Dark matter is generally thought to form the basis of all galaxies, gravitationally attracting gas, which eventually forms stars. So some process must have separated dark matter from the gas of galaxies.
Scientists have previously seen dark matter and normal matter split on a very large scale in the Bullet Cluster, which formed when two clusters of galaxies collided with each other (SN: 23.08.06). Other researchers have suggested that something similar could happen to colliding dwarf galaxies, what van Docum and his colleagues call “dwarf bullets.”
In such a collision, the etheric dark matter of the dwarf galaxies will continue unperturbed, because the dark matter does not interact with other matter. But the gas from the two galaxies will collide together, eventually forming multiple clumps, each of which will become its own galaxy without dark matter.
Now Van Docum and his colleagues say the idea of ​​the dwarf bullet explains the two previously reported galaxies without dark matter – and several other galaxies nearby. The two galaxies are moving away from each other as if they came from the same place, researchers say. Moreover, the two galaxies are part of a chain of 11 galaxies arranged in a row, a structure that may have formed after a bullet collided with a dwarf.
“It’s super satisfying to finally have an explanation for these strange objects,” van Docum said.
But, says Collins, “a lot more can be done to make it convincing.” For example, she says, scientists have not measured the distances of all galaxies on Earth. This means that some of the galaxies may be much farther away from others, and it may be a coincidence that the galaxies appear to be ordered from our point of view.
And researchers have not yet measured the velocities of all the galaxies in the trail or determined whether those galaxies also lack dark matter, which would help confirm whether the scenario is correct.
Other scientists are more optimistic. “The origin story is very plausible in my opinion,” said astrophysicist Yun-jin Shin of Seoul National University in South Korea. Shin wrote perspective article on the discovery with astrophysicist Ji Hong Kim, also from Seoul National University, which was also published in nature.
Computer simulations by Shin, Kim, and others have shown that bullet dwarves can produce such galaxies without dark matter. If confirmed, the idea of ​​the dwarf bullet can help determine the properties of dark matter, in particular whether dark matter interacts with itself (SN: 4/5/18).
Van Docum and his colleagues are planning additional measurements that could confirm or disprove the case. But so far he says, “For me, this has a ring of truth.”
A galactic smashup might explain galaxies without dark matter
