The galaxy lacks dark matter – how is it held together?
A research group has found a galaxy that seems to lack dark matter. The discovery goes against previous theories about how galaxies are built.
What has happened?
A research team has discovered a galaxy, AGC 114905, without dark matter. The galaxy, which is 250 million light-years away, has been closely studied using the Very Large Array Observatory (VLA) in New Mexico. The 27 large satellite telescopes that make up the VLA were aimed at the galaxy for 40 hours to accurately measure its rotational speed.
What role does dark matter play in galaxies?
The idea of dark matter has its place origin in how galaxies rotate.
In general, the visible mass, including stars, is not sufficient for galaxies to be held together. The rotation of an ordinary galaxy means that the stars should fly away from each other, and in order for them to be held together, there must be something – invisible to us – that contributes mass. This is something called dark matter.
The Very Large Array Observatory in New Mexico consists of 27 large satellite dishes telescopes set up in a Y formation. Photo: Science Photo Library / TT
What dark matter consists of is not known, but it is thought to make up 27 percent of the universe. Ordinary matter accounts for a paltry five percent and the remaining 68 percent is dark energy, an energy that is linked to the expansion of the universe.
The galaxy AGC 114905 thus has no dark matter based on the measurements that have been made, which is very unusual.
How is AGC 114905 kept together if dark matter is missing?
In the recently published the study The researchers explain why dark matter is not needed to hold AGC 114905 together. To detect dark matter, the rotation of gas clouds around the center of the galaxy has been studied. The velocities measured are usually so high that a proportion of dark matter must be added so that the galaxy does not dissolve. But for AGC 114905, the visible matter is sufficient to explain the rotation measured.
How did astronomers know where to look?
During 2019 was observed six galaxies that were unusually diffuse, low-brightness galaxies. The observations also indicated that the galaxies lacked dark matter. To confirm these first measurements, they chose to point the telescope in VLA at the galaxy AGC 114905, and as the researchers hoped, no dark matter was discovered.
– This is exactly what we believed and hoped for because it confirms our previous measurements, says Pavel Mancera Piña, one of the astronomers behind the study, in a press release.
Do you know why the galaxy lacks dark matter?
One theory is that gravity from larger galaxies can attract dark matter from other galaxies. But according to Pavel Mancera Piña, there are no large enough galaxies near AGC 114905 to accomplish this. The other theories presented in the study also cannot explain why the galaxy has no dark matter.
The observed galaxy is also angled from Earth. In order to be able to produce data on how it rotates, researchers have had to estimate how large that angle is. If the estimated angle is wrong, it could affect the researchers’ conclusion.
– But that angle must deviate very much from our estimates before there is room for dark matter again, says co-author Tom Oosterloo in the press release.
Are there other galaxies that lack dark matter?
The first example of a galaxy that could lack dark matter is NGC 1052-DF2. But according to the press release, the measurements of AGC 114905 are more robust. In order to be able to draw proper conclusions, more measurement data is needed and the researchers are now investigating one more of the six galaxies that they became interested in in 2019.
The Swede who was the first with the dark matter
The credit for discovering dark matter is usually attributed to Fritz Zwicky, the Swiss astronomer who, while studying galaxy clusters, discovered that something was missing. In 1933 he showed that the Comahop needed to have a much larger mass than was apparent in order for the rotation to be reasonable.
In 2015 was found however, an article that shows that the Swedish astronomer Knut Lundmark was a few years earlier than Zwicky in proving the dark matter, which Dagens Nyheter was able to reveal.
By estimating the visible matter and looking at the rotation, he showed that several galaxies needed to contain a large amount of invisible matter. Other pioneers in dark matter research include astronomers Vera Rubin and Kent Ford. Read more about it in Ny Teknik’s article here.
