Astronomers believe that galaxies cannot form without the gravitational pull of dark matter. Therefore, finding a trail of galaxies where this mysterious substance was not present, without an obvious cause, would be an extraordinary find.
In an article published in Nature On May 18, a team of astronomers claims to have observed such a system: a row of 11 galaxies that do not contain dark matter and that could have been created in the same collision. But many of his colleagues are not convinced that this claim is more than a mere hypothesis.
This type of system could help understand how galaxies are formed and what is the nature of dark matter itself. “If confirmed, this finding could be very interesting for galaxy formation. However, it remains to be seen if this is true”, says Chervin Laporte, an astronomer at the University of Barcelona.
The finding focuses on two galaxies described in 2018 and 2019 by Pieter van Dokkum of Yale University and his collaborators. Their stars moved so slowly that dark matter attraction was not needed to explain their orbits, so the team concluded that galaxies did not contain dark matter.
The conclusion was highly controversial because those galaxies, called NGC 1052-DF2 and NGC 1052-DF4 (DF2 and DF4, from now on), seemed stable and distinct from the only other galaxies without dark matter that we know of. The latter are recent and short-lived, appearing in the arms of larger galaxies whose dark matter is being stripped away by another nearby object. But how DF2 and DF4 were formed was something of a mystery.
In their recent article, Van Dokkum’s team not only establishes a relationship between the two unusual galaxies, but also states that their properties suggest that they were formed in a high-speed collision 8 billion years ago and would have given rise to other galaxies. structures of the same type. “This possibility explains many strange properties of these galaxies,” says Van Dokkum.
The team deduced that scenario from simulations created to explain certain unique aspects of larger-scale collisions between galaxy clusters. The researchers propose that two parent galaxies collided head-on, so that their dark matter and stars intersected.
The dark matter would not have interacted and the stars were surely too spread out to collide. But, while these components were moving at full speed, the gas between the stars of the two galaxies would have collided, compacted and slowed down, leaving a trail of matter that later formed new galaxies without dark matter.
Next, the researchers looked for such galaxies along the line through DF2 and DF4. They identified between three and seven new galaxy candidates without dark matter, as well as two strange, faint galaxies at the edges, which could correspond to the dark matter and stars left over from the parent galaxies. “Once we knew what to look for, it was right under our noses,” shares Van Dokkum.
If that idea is confirmed, it would help astronomers understand how dark matter behaves and the circumstances in which galaxies can form. Such a galactic collision could also serve as a “new laboratory” to find out if dark matter interacts with itself, says Go Ogiya, an astronomer at Zhejiang University.
While plausible, Van Dokkum’s model describes only one of the ways these galaxies could have formed, stresses Priyamvada Natarajan, an astrophysicist at Yale who is not on Van Dokkum’s team. But she gives food for thought and, above all, makes testable predictions, she values.
Precisely measuring the distances and velocities of the candidate galaxies could show that they are part of the same chain and rule out that they happen to be in the same line of sight, says Michelle Collins, an astronomer at the University of Surrey. “A big open question is whether or not this is a true line.”
Laporte adds that astronomers also need to measure the masses of the faint galaxies at the ends of the line—the possible parent galaxies—to see if they contain as much dark matter as the model predicts.
Others question the need for an exotic explanation. Ignacio Trujillo, an astronomer at the Instituto de Astrofísica de Canarias, leads a team that proposes that DF2 and DF4 are closer to Earth than the Van Dokkum measurements indicate. As a consequence, these galaxies would contain more dark matter than first appeared.
Astronomers also need a reliable simulation that shows that the scenario Van Dokkum’s team describes is feasible, says Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore. “At the moment there are many assumptions, but we do not have any simulation to support them,” she concludes.
Article translated and adapted by Research and Science with permission from Nature Research Group.
Reference: “A trail of dark-matter-free galaxies from a bullet-dwarf collision”, Pieter van Dokkum et al. in Nature, vol. 605, pp. 435-439, May 19, 2022.
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