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

This type of system may very well be used to study how galaxies type, and concerning the nature of darkish matter itself. “If confirmed proper, this might actually be thrilling for galaxy formation. Nonetheless, the jury continues 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 elucidate their orbits, so the crew concluded that the galaxies contained no darkish matter. The discovering was controversial as a result of the galaxies, named DF2 and DF4, appeared secure and totally different from the one different identified 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 fashioned was a thriller.

Telltale path

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

The crew borrowed its situation from simulations initially created to elucidate 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 celebs would have been too far aside to collide. However because the darkish matter and stars sped on, fuel within the area between the 2 galaxies’ stars would have crashed collectively, compacted and slowed down, leaving a path of matter that later fashioned new galaxies with no darkish matter.

Subsequent, the researchers appeared 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 may assist astronomers to know how darkish matter behaves, and to be taught concerning the circumstances below which galaxies can type. Such a galactic collision may additionally be used as a “new laboratory” to know 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 various ways in which these galaxies may have been made, says Priyamvada Natarajan, an astrophysicist at Yale who will not be a member of van Dokkum’s crew. However it’s intriguing and, crucially, makes testable predictions, she says.

Measurements of the exact distances and velocities of candidate galaxies may 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 this can be 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 crew 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 crew describes is believable, says Mireia Montes, an astronomer on the House Telescope Science Institute in Baltimore, Maryland. “For now, there are quite a lot of assumptions, however these should not supported by any simulation,” she says.

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