Astronomers believe that galaxies cannot form without the gravitational pull of dark matter. So a trail of galaxies free of this mysterious material, with no obvious cause, would be a remarkable find. In an article published in Nature May 181, astronomers say they could have observed such a system – a line of 11 galaxies that contain no dark matter, all of which may have been created in the same ancient collision. But many of their peers are not convinced that this claim is much more than a hypothesis.
This kind of system could be used to learn more about galaxy formation and the nature of dark matter itself. “If this turns out to be correct, it could certainly be exciting for galaxy formation. However, the jury is still out,” says Chervin Laporte, an astronomer at the University of Barcelona in Spain.
The discovery focuses on two galaxies described by Pieter van Dokkum of Yale University in New Haven, Connecticut, and his co-authors in 2018 and 2019. Their stars moved so slowly that the attraction of dark matter did not was not necessary to explain their orbits, so the team concluded that the galaxies did not contain dark matter. The discovery was controversial because the galaxies, named DF2 and DF4, appeared stable and different from the only other known galaxies without dark matter, which are new and short-lived, created in the arms of larger galaxies stripped of dark matter. by a neighbor. The formation of DF2 and DF4 was a mystery.
In the latest paper, van Dokkum’s team not only connects the two unusual galaxies, but claims that their properties are consistent with their formation in a high-speed collision eight billion years ago, which also generated other structures of this type. “This single explanation explains so many strange things about these galaxies,” van Dokkum says.
The team borrowed their scenario from simulations originally created to explain the unique characteristics of larger-scale collisions between galaxy clusters. The researchers suggest that when two progenitor galaxies collided head-on, their dark matter and stars would have crossed paths; dark matter would not have interacted and the stars would have been too far apart to collide. But as the dark matter and stars accelerated, the gas in the space between the stars of the two galaxies would have crashed together, compacted and slowed down, leaving a trail of matter that then formed new galaxies without dark matter.
Next, the researchers searched for such galaxies in the line between DF2 and DF4. They identified between three and seven new candidates for galaxies without dark matter, as well as strange, faint galaxies at either end, which could be the dark matter and remnant stars of the progenitor galaxies. “It stared you in the face once you knew what to look for,” van Dokkum says.
If this image turns out to be true, it could help astronomers understand the behavior of dark matter and learn more about the circumstances in which galaxies can form. Such a galactic collision could also be used as a “new laboratory” to understand whether dark matter interacts with itself, says Go Ogiya, an astronomer at Zhejiang University in Hangzhou, China.
While plausible, van Dokkum’s model only describes one of many ways these galaxies could have been made, says Priyamvada Natarajan, an astrophysicist at Yale who is not part of van Dokkum’s team. But it’s intriguing and, most importantly, makes testable predictions, she says.
Measurements of the precise distances and velocities of candidate galaxies could prove that they are part of the same chain and not just coincidentally along the same line of sight, says Michelle Collins, an astronomer at the University of Surrey in Guildford, UK. “To me, whether this is a real line or not is a big open question.”
Astronomers also need to measure the masses of ghost galaxies at the ends of the line — potential progenitor galaxies — to test whether they contain a lot of dark matter, as the model predicts, Laporte adds.
Others wonder if an exotic explanation is necessary. Ignacio Trujillo, an astronomer at the Canary Islands Institute of Astrophysics in La Laguna, Spain, leads a team that proposes that DF2 and DF4 are closer to Earth than van Dokkum’s measurements suggest, and therefore contain more dark matter than it appears at first glance. .
Astronomers also need to see a reliable simulation showing that the scenario van Dokkum’s team describes is plausible, says Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. “At the moment there are a lot of assumptions, but these are not supported by any simulation,” she says.