Galaxies without dark matter perplex astronomers

An image of the NGC 1052 galaxy captured by the Hubble Telescope

This Hubble picture captures a set of galaxies which are uncommon as a result of they appear to not have darkish matter.Credit score: NASA/ESA/P. van Dokkum, Yale Univ.

Astronomers suppose that galaxies can not kind with out the gravitational pull of darkish matter. So a path of galaxies freed from this mysterious materials, with no apparent trigger, could be a exceptional discover. In a paper printed in Nature on 18 Could1, astronomers say they may have noticed such a system — a line of 11 galaxies that don’t comprise any darkish matter, which might all have been created in the identical historic collision. However a lot of their friends are unconvinced that the declare is way more than a speculation.

This sort of system may very well be used to find out about how galaxies kind, and in regards to the nature of darkish matter itself. “If confirmed proper, this might actually be thrilling for galaxy formation. Nevertheless, the jury remains to be out,” says Chervin Laporte, an astronomer on the College of Barcelona in Spain.

The discovering centres on two galaxies described by Pieter van Dokkum at Yale College in New Haven, Connecticut, and his co-authors in 2018 and 2019. Their stars moved so slowly that the pull of darkish matter was not wanted to clarify their orbits, so the workforce concluded that the galaxies contained no darkish matter. The discovering was controversial as a result of the galaxies, named DF2 and DF4, appeared steady and completely different from the one different recognized dark-matter-free galaxies, that are new and short-lived, created within the arms of bigger galaxies whose darkish matter is being stripped by a neighbour. How DF2 and DF4 shaped was a thriller.

Telltale path

Within the newest paper, van Dokkum’s workforce not solely connects the 2 uncommon galaxies, however says their properties are in step with them being shaped in a high-speed collision, eight billion years in the past, that additionally spawned extra such constructions. “This single rationalization explains so many odd issues about these galaxies,” says van Dokkum.

The workforce borrowed its situation from simulations initially created to clarify distinctive options in larger-scale collisions between galaxy clusters. The researchers recommend that when two progenitor galaxies collided head on, their darkish matter and stars would have sailed previous one another; the darkish matter wouldn’t have interacted, and the celebrities would have been too far aside to collide. However because the darkish matter and stars sped on, gasoline within the house between the 2 galaxies’ stars would have crashed collectively, compacted and slowed down, leaving a path of matter that later shaped new galaxies with no darkish matter.

Subsequent, the researchers regarded for such galaxies within the line between DF2 and DF4. They recognized between three and 7 new candidates for dark-matter-free galaxies, in addition to unusual, faint galaxies at both finish, which may very well be the darkish matter and stars remaining from the progenitor galaxies. “It was staring you within the face when you knew what to search for,” says van Dokkum.

If this image proves to be true, it might assist astronomers to grasp how darkish matter behaves, and to study in regards to the circumstances beneath which galaxies can kind. Such a galactic collision may additionally be used as a “new laboratory” to grasp whether or not darkish matter interacts with itself, says Go Ogiya, an astronomer at Zhejiang College in Hangzhou, China.

Open questions

Though believable, van Dokkum’s mannequin describes simply one among quite a lot of ways in which these galaxies might have been made, says Priyamvada Natarajan, an astrophysicist at Yale who just isn’t a member of van Dokkum’s workforce. However it’s intriguing and, crucially, makes testable predictions, she says.

Measurements of the exact distances and velocities of candidate galaxies might show they’re a part of the identical string and never simply coincidentally alongside the identical line of sight, says Michelle Collins, an astronomer on the College of Surrey in Guildford, UK. “For me, whether or not it is a actual line or not is a giant open query.”

Astronomers additionally must measure the plenty of the ghost galaxies on the ends of the road — the potential progenitor galaxies — to check whether or not they comprise a number of darkish matter, because the mannequin predicts, provides Laporte.

Others query whether or not any unique rationalization is required. Ignacio Trujillo, an astronomer on the Institute of Astrophysics of the Canary Islands in La Laguna, Spain, leads a workforce that proposes that DF2 and DF4 are nearer to Earth than van Dokkum’s measurements recommend, and due to this fact comprise extra darkish matter than was at first obvious.

Astronomers additionally must see a dependable simulation exhibiting that the situation van Dokkum’s workforce describes is believable, says Mireia Montes, an astronomer on the House Telescope Science Institute in Baltimore, Maryland. “For now, there are numerous assumptions, however these are usually not supported by any simulation,” she says.

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