Astronomers discover a second galaxy without black matter

A year ago, astronomers announced their surprise discovery of a galaxy almost entirely devoid of dark matter. While the first galaxy had never found the elusive substance (which would represent 85% of the mass of the universe), the news spread throughout the astronomical community. This has left some researchers deliciously intrigued, and others naturally skeptical.

"If there is [only] an object, you always have a small voice in the back of your mind that says, "but what if you're wrong?" "Said the astronomer Pieter van Dokkum of Yale University, who had led the groundbreaking study of last year, in a press release. "Even though we did all the checks we could think of, we were worried that nature had left us out and that it conspired to give something really special when it was really something more banal. "

Now, a new study published in The letters of the astrophysical journal On March 27, van Dokkum and his team were right.

According to the works, they confirmed that a ghostly galaxy located about 60 million light-years away from the planet, named NGC 1052-DF2 (DF2 abbreviated), contains practically no noticeable dark matter. In addition, a second study published March 20 in the same newspaper announced the discovery of another galaxy weak and diffuse with a shortage of dark matter, nicknamed DF4.

Taken together, the new documents show that DF2 is not alone, but is part of a larger and then unknown population of galaxies that seem to be released from the links of matter. black. This new research could have dramatic implications for the prevailing theories about the formation and evolution of galaxies, as well as the true nature of dark matter itself.

"Seeing something completely new is really fascinating," said Shany Danieli, a graduate student at Yale University and senior author of the latest DF2 study. "Nobody knew that such galaxies existed, and the best thing for an astronomy student in the world is to discover an object – be it a planet, a star, or a galaxy – that nobody knew or thought of anything. is."

Discovering an Oddball galaxy

The strange galaxy DF2 belongs to a relatively new class of galaxies called ultra-diffuse galaxies (UDG). Although UDGs can reach the size of the Milky Way, these weak spectra contain hundreds, if not thousands, fewer stars. This means that you can really see through, which makes the UDG very difficult to observe in detail.

DF2 is a hazy galaxy belonging to a larger group dominated by the gigantic elliptical galaxy NGC 1052. Initially, the researchers were attracted to the galaxy harmless, because it did not appear in the same way in the images captured by the telephoto Dragonfly and those obtained by the Sloan Digital Sky Survey (SDSS). Dragonfly viewed DF2 as a drop of dim light, while SDSS saw a group of point sources.

After a closer look, use Hubble's Advanced Camera for Surveys and the 10-meter W.M. According to the Keck Observatory, the researchers determined that DF2 is a transparent UDG that rivals the Milky Way in size but contains about 200 times fewer stars.

"I spent an hour looking at the Hubble image," van Dokkum said in a press release last year. "It's so rare, especially these days after so many years at Hubble, that you get a picture of something and you say, 'I've never seen this before.' This thing is amazing: a gigantic spot in which you can look. It's so rare that you see all the galaxies behind. "

With the new DF2 views, van Dokkum and his team identified 10 globular clusters (large old star groups) inside the galaxy and discovered that they were moving three times slower provided that. This suggests that DF2 contains, at most, only a tiny amount of dark matter. After all, if it had more mass than what was visible, the bunches would gravitate much faster.

A second look at a ghost

One of the major principles of good science being reproducibility, van Dokkum and his team spent the last year using the Keck Cosmic Web Imager (KCWI) to improve the accuracy of their previous measurements.

"KCWI is unique because of the combination of its vast survey area," Danieli said. "The instrument not only allows us to see the galaxy at the same time, but its high spectral resolution also allows us to measure mass accurately. There is no other instrument in the world that has these two properties. "

Armed with even more accurate measurements of the velocity of globular clusters within DF2, the researchers again calculated the mass of the galaxy. As before, their results show that the amount of dark matter in DF2 is practically nil.

Double

While seeking to confirm that DF2 is truly a galaxy deficient in dark matter, the researchers were simultaneously looking for a second example of a galaxy made exclusively from normal matter. And they found it in DF4.

"Discovering a second galaxy with very little or no dark matter is just as exciting as the initial discovery of DF2," said van Dokkum, lead author of the article on DF4. "This means that the chances of finding more of these galaxies are now higher than expected. As 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. "

According to the study, DF2 and DF4 are very similar in terms of size, brightness, morphology and distance. Because of this, the paper says, "We conclude that NGC 1052-DF2 is not an isolated case but that there is a class of objects of this type. The origin of these large weak galaxies with an excess of bright globular clusters and an apparent lack of dark matter is, to date, unknown. "

Implication for dark matter

With a sample of only two galaxies, it is difficult to draw radical conclusions for the moment. But by showing that some galaxies do not contain appreciable dark matter, the team has surprisingly provided solid evidence of dark matter.

Since the effects of dark matter are evident in all other known galaxies, except for DF2 and DF4, the team's findings indicate that dark matter must be a tangible substance, separable from normal matter. This means that some alternative theories of dark matter, such as Modified Newtonian Dynamics (MOND) – which introduces additional gravitational forces on the galactic scales – fall flat.

"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. "