Our understanding of the universe, at its largest scales, relies to some extent on things we cannot see. Therefore, the discovery of a galaxy without dark matter (or with a very small amount) is proving disconcerting.

Finding a galaxy without dark matter is, in a way, against the rules. It's like watching a group of seaweeds grow on top of Everest. The general picture is that galaxies, like the Milky Way, are formed within accumulations of dark matter. This type of matter, which we cannot see and which accounts for about 20% of the composition of the universe, plays an important role.

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The galaxies are surrounded by a halo of dark matter. The presence of this matter can be deduced from several factors. Like the fact that, without it, it is difficult to explain why a galaxy can be held together by its own gravity. The stars that compose it would end up being thrown in all directions. So the discovery of NGC 1052-DF2 is extremely striking.

But let us put ourselves in context, because there are some clarifications to be made. NGC 1052-DF2 is a satellite galaxy of NGC 1052. The latter is a giant elliptical galaxy. It's surrounded by satellite galaxies. It's the same with ours. Among the satellites of the Milky Way are the Magellanic Clouds, which are visible from the southern hemisphere of the Earth. This is where the Dragonfly telescope comes in.

###Dragonfly, a special telescope

To be able to observe these satellite galaxies, which are usually much fainter than the main galaxies they accompany, a simple telescope is not enough. In this case, the Dragonfly telescope has been used, which is very particular. Because, technically speaking, it's not a telescope in the strict sense. It is a set of Canon photo lenses, connected to a digital detector and installed in the same frame so that they point in the same direction.

The lenses have a special coating to reduce light dispersion. This helps to see large, faint and diffuse objects. In a traditional telescope, these objects disappear under the glare of stars and other brighter objects. Dragonfly reduces that interfering light, and can also observe large regions of the sky. So you can see many things at once. Dragonfly's main mission is to take images of nearby galaxies in search of satellites that go unnoticed.

That's how we came to the discovery of NGC 1052-DF2. But to find out it's a galaxy without dark matter would take more than that. With the image quality of Dragonfly they were already able to detect that it has many points of light. But in order to better observe it, they decided to turn to the Hubble telescope for help. The result is tremendously intriguing, because not only is it a diffuse galaxy.

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The intriguing NGC 1052-DF2

In the first image we can see the galaxy in detail (left side of the image). It's that blue region in the center of the image. Almost everything else we can see is more distant galaxies. Some of them are even visible through NGC 1052-DF2. At first glance, it is striking that there seems to be no gas. No nebula visible. The places where new stars are formed.

We can also see many of the stars that seem to make up the galaxy. Although, to tell you the truth, not all of them are stars. Some of the dots, brighter than the others, are actually globular clusters. They are groups of thousands of stars, which are held together by their combined gravity. These globular clusters are striking because they are brighter than most we can see, even here in the Milky Way.

Thanks to the Keck Observatory (in Hawaii), researchers were able to analyze NGC 1052-DF2 in more detail. These globular clusters are the ones that, to a large extent, tell us the mystery of this galaxy without dark matter. From the Keck observatory, the light from NGC 1052-DF2 was analyzed and separated into its individual colors. In this way, a lot of information can be extracted. It not only allows us to determine that they are groups of stars.

The history of globular clusters

By analyzing the light they emit, it is possible to measure their speed. It's a very useful thing. Because the speed at which they move depends on the gravity they are subjected to in the galaxy they are in. This allows us to determine, by extension, the mass of the galaxy. When analyzing the velocity distribution of the NGC 1052-DF2 globular clusters, astronomers determined that it has no more than 320 million times the mass of the Sun and is probably closer to 200 million.

If we look at all the light emitted by the galaxy, it is also possible to determine the total mass of all the stars in it. And here comes the surprise. The estimate is about 200 million times the mass of the Sun. Researchers have used several methods to measure the mass of NGC 1052-DF2, and all give about the same figure. That is, the entire mass of the galaxy is found in stars.

It's quite a dilemma. So far, the galaxies we've observed have a corona of dark matter. We know that it is about 6 times more abundant than normal matter and it has always been assumed that it is essential to allow the formation of galaxies. The final amount of dark matter that a galaxy will have around it depends on many factors, such as the number of stars. But in the case of dwarf galaxies like this one, it's even more extreme.

A galaxy without dark matter that goes against everything

Because in the case of dwarf galaxies, what is expected is that the difference between their normal matter and their dark matter will be several hundred times greater. However, far from showing such a proportion, what we see is NGC 1052-DF2 seems to have nothing. It's a galaxy without dark matter. Or, at best, it may have as much dark matter as normal matter. Something that doesn't fit what we know.

What happened to that dark matter? It's hard to answer that question. Maybe he didn't have a dark matter crown around him. It may have formed from gas lost by NGC 1052, the nearby giant elliptical galaxy. The latter could happen when two large galaxies collide. In addition, it is believed that elliptical galaxies form precisely after the collision of two galaxies. Or maybe that gas was expelled through the supermassive black hole in the center of NGC 1052.

Another possibility is that the gas that fell into NGC 1052 could form stars before it reached the giant galaxy. But no matter how you look at it, NGC 1052-DF2 raises four big questions. It's bigger than most dwarf galaxies. It is also more diffuse. Their globular clusters are brighter than normal and it's a galaxy without dark matter. What conclusion can be drawn from all this?

It's too early to draw conclusions

There's not much to say at the moment. NGC 1052-DF2 is a galaxy without dark matter or, at best, much less than it should be. But to better understand its nature there are only two possibilities. Study it in more detail or look for other dwarf galaxies that exhibit similar behavior. The objective of the Dragonfly telescope is to find other diffuse dwarf galaxies.

You will probably find many dwarf galaxies that accompany other large galaxies. What's not so clear is that they're going to find something as striking as NGC 1052-DF2. Are there often galaxies without dark matter? Discovering it will teach us something about the universe we live in. Dragonfly has already helped to study ultra-diffuse galaxies, which stand out for their low brightness.

How does the discovery of a galaxy without dark matter affect our conception of the universe? For the moment, it's something that should make us arch our eyebrows. It's strange and our understanding of the universe tells us it shouldn't be there. But until we know of other similar objects, or are studied in more detail, it would be hasty to draw conclusions. We'll have to wait and see what we get in the near future. Because this will certainly not be the last time we will talk about this subject.