Remember from that time that astronomers discovered a galaxy without dark matter? Well, they're back. They found a second, and this really reinforces the case. for existence of dark matter.
As the astronomers of the second galaxy discovered without any dark matter, the new discovery – called NGC 1052-DF4 (abbreviated as DF4) – confirms that the first discovery, NGC 1052-DF2 (yes, DF2), was not an error.
After its discovery, the DF2 was indeed a great surprise, and cast a very heavy key in our current ideas about the formation and dynamics of galaxies because dark matter is a vital part of our understanding of galaxies.
The strange thing is currently undetectable even for our best instruments, but we know there is something out there, some invisible mass, increasing the gravitational forces at play in the galaxies. In the Milky Way, for example, the velocity of the outer edge of the galaxy is much faster than it would be if it were affected only by the detectable matter.
In some galaxies, there seems to be more dark matter than normal matter; and until the discovery of DF2, it was thought that dark matter is not just a component, but a requirement for galaxies to form in the first place.
Thus, this initial article attracted some criticism – and even some doubts among the team.
"If there is an object, you always have a little voice in the back of your mind saying," but what if you're wrong? ", Said astronomer Pieter van Dokkum of Yale University.
"Although we had done all the tests we could think of, we were worried that nature had thrown us into a cycle and conspired to make something look really special while it was really something more mundane."
So they found DF4. Like DF2, it is an ultra-diffuse galaxy – fairly large, scattered, and weak to be observed. These objects are about the size of the Milky Way, but with 100 to 1,000 times fewer stars, so they can be quite difficult to see.
Both galaxies were also associated with the elliptical galaxy NGC 1052, about 63 million light-years away from the constellation Cetus.
And like DF2, DF4 seems to be completely devoid of dark matter. Using the Keck Observatory's Low Resolution Image Spectrometer (LRIS), astronomers tracked the orbital motion of seven dense clusters of stars called globular clusters. Its velocity turned out to be consistent with the gravitational effect of the galaxy's estimated mass of normal matter.
"Discovering a second galaxy with little or no dark matter is 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 previously thought. Since we have no good ideas of how these galaxies were formed, I hope these findings will encourage more scientists to work on this enigma."
The team also carried out further research to confirm previous DF2 results. Using the powerful Keck Cosmic Web Imager (KCWI) from the W. Keck Observatory, they tracked the orbits of 10 globular clumps. Their speed was also consistent with the lack of dark matter.
And because these galaxies are so peculiarly dark without matter, they are in fact a supporting argument for their existence – because they prove that normal matter can exist separately.
There are, of course, alternative hypotheses to the theory of dark matter; but under these aspects, the existence of these galaxies becomes a more difficult problem, the researchers said.
As for why these galaxies do not have dark matter, this remains unsolved. Have they had and lost in any way? Did they form without it the gas ejected by NGC 1052? It's hard to say – but maybe if we find even more galaxies like this, we could start putting them together.
"We then hope to find out how these galaxies are common and whether they exist in other areas of the universe," said astronomer Shany Danieli of Yale University.
"We want to find more evidence to help us understand how the properties of these galaxies work with our current theories." Our hope is that it will take us a step further in understanding one of the greatest mysteries of our universe – the nature of darkness.
The research was published in The Astrophysical Journal Letters here and here