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Galaxies without dark matter perplex astronomers - Nature.com

Galaxies without dark matter perplex astronomers - Nature.com

Galaxies without dark matter perplex astronomers - Nature.com
May 19, 2022 2 mins, 12 secs

This Hubble image captures a set of galaxies that are unusual because they seem not to have dark matter.Credit: NASA/ESA/P.

Astronomers think that galaxies cannot form without the gravitational pull of dark matter.

In a paper published in Nature on 18 May1, astronomers say they might have observed such a system — a line of 11 galaxies that don’t contain any dark matter, which could all have been created in the same ancient collision.

This kind of system could be used to learn about how galaxies form, and about the nature of dark matter itself.

Their stars moved so slowly that the pull of dark matter was not needed to explain their orbits, so the team concluded that the galaxies contained no dark matter.

The finding was controversial because the galaxies, named DF2 and DF4, seemed stable and different from the only other known dark-matter-free galaxies, which are new and short-lived, created in the arms of larger galaxies whose dark matter is being stripped by a neighbour.

In the latest paper, van Dokkum’s team not only connects the two unusual galaxies, but says their properties are consistent with them being formed in a high-speed collision, eight billion years ago, that also spawned more such structures.

The researchers suggest that when two progenitor galaxies collided head on, their dark matter and stars would have sailed past each other; the dark matter would not have interacted, and the stars would have been too far apart to collide.

But as the dark matter and stars sped on, gas in the space between the two galaxies’ stars would have crashed together, compacted and slowed down, leaving a trail of matter that later formed new galaxies with no dark matter.

They identified between three and seven new candidates for dark-matter-free galaxies, as well as strange, faint galaxies at either end, which could be the dark matter and stars remaining from the progenitor galaxies.

If this picture proves to be true, it could help astronomers to understand how dark matter behaves, and to learn about the circumstances under which galaxies can form.

Measurements of the precise distances and velocities of candidate galaxies could prove they are part of the same string and not just coincidentally along the same line of sight, says Michelle Collins, an astronomer at the University of Surrey in Guildford, UK.

Astronomers also need to measure the masses of the ghost galaxies at the ends of the line — the potential progenitor galaxies — to test whether they contain lots of dark matter, as the model predicts, adds Laporte.

Read the related News & Views: ‘Giant collision created galaxies devoid of dark matter’

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