Astronomers confirm the existence of galaxies without dark matter

A team of researchers led by Yale doubles his conclusions about the discovery of a galaxy almost devoid of dark matter.

In 2018, researchers published their original study on the galaxy NGC 1052-DF2-DF2 – the first known galaxy to contain little or no dark matter. The discovery was highly significant because it showed that dark matter was not always associated with the traditional material at the galactic scale. He also excluded several theories according to which dark matter is not a substance but a manifestation of the laws of gravity on a cosmic scale.

Invisible dark matter generally dominates the composition of galaxies. Finding an object without dark matter was unprecedented and sparked many debates within the scientific community.

"If there is only one object, you always have a small voice in mind that says," But if you are wrong? ", Said team leader Pieter van Dokkum, professor of family astronomy Sol Goldman at Yale." Although we did all the checks we could think of, we were worried that nature would left out and that he conspired to give something really special when it was really something more banal. "

Two new studies published in The Astrophysical Journal Letters support the first conclusions of the team.

"Seeing something completely new is really fascinating," said Shany Danieli, a Yale graduate student who first discovered the galaxy about two years ago. "Nobody knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, be it a planet, a star, or a galaxy, that no one knew or thought of anything. is."

Danieli is the main author of one of the new studies. It confirms the first observations of DF2 by the team, using more precise WM measurements. Keck Cosmic Web Imager from the Keck Observatory. The researchers found that stars inside the galaxy move at a speed consistent with the mass of normal matter in the galaxy. If there was dark matter in DF2, the stars would move much faster.

Van Dokkum is the main author of the new study, which details the discovery of a second galaxy devoid of dark matter. This galaxy is called DF4.

"The discovery of a second galaxy with very little or no dark matter is just as exciting as the initial discovery of DF2," van Dokkum said. "This means that the chances of finding more of these galaxies are now higher than expected. Since we do not have good ideas on the formation of these galaxies, I hope that these discoveries will encourage more scientists to work on this puzzle. "

Both DF2 and DF4 are part of a relatively new class of galaxies called ultra-diffuse galaxies (UDGs). They are as big as the Milky Way but have between 100 and 1000 times less stars. This gives them a soft, translucent appearance – and hard to see.

Ironically, the lack of dark matter in these UDGs reinforces the arguments for dark matter, say the researchers. This proves that dark matter is a substance that is not associated with normal matter, since they can be found separately.

In addition to Van Dokkum and Danieli, members of the team include Roberto Abraham of the University of Toronto, Aaron Romanowsky of San Jose State University and Charlie Conroy of Harvard.

Danieli is leading a large-scale investigation with the Dragonfly Telephoto Array – a telescope designed by van Dokkum – to search for more examples in a systematic way, and then re-examine the candidates with the help of Keck telescopes.

"We hope to find out how common these galaxies are and whether they exist in other parts of the universe," Danieli said. "We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories. We hope this will allow us to go even further in understanding one of the greatest mysteries of our universe – the nature of dark matter. "

Publications:

• Shany Danieli, et al., "Dark matter still missing: KCWI high-resolution star kinematics of NGC1052-DF2", ApJL, 2019; doi: 10.3847 / 2041-8213 / ab0e8c
• Pieter van Dokkum. et al., "A second galaxy lacking dark matter in NGC 1052", ApJL, 2019; doi: 10.3847 / 2041-8213 / ab0d92