An enigma for astronomers: DF2 - the galaxy without dark matter

The recent discovery of a galaxy with dimensions similar to ours, which does not contain dark matter, astonished astronomers who were convinced that all galaxies contained this bizarre form of matter made up of particles still unknown. DF2 Galaxy raises questions about how galaxies are formed. As it seems bizarre, its existence strengthens astronomers' belief that ... dark matter exists.



Up to now, it was believed that all galaxies - at least those of significant size - contain a much darker matter much larger than the amount of ordinary matter.

Known matter is composed of stars, planets, dust and interstellar gas, but which fails to explain the rotation speed of stars in the galaxy's periphery, which depends on the gravitational pull of matter in the galaxy. This speed is much higher than it should be if only known matter is responsible for the gravitational pull. Thus, the idea of ​​dark matter - the one responsible for gravitational attraction "in addition" - was introduced in physics.

Our galaxy has, for example, a dark matter quantity about 30 times greater than known matter. At the level of the entire universe, it is believed that dark matter is about five times more than the matter we know. Not all are convinced of the existence of this matter, and argue that the solution would be to modify the laws of gravity attraction at great distances (so-called MOND-Modified Newtonian Dynamics).

In this context, the recent discovery of a galaxy, NGC 1052-DF2 (short DF2) with the Dragonfly Telescope Array telescope, which was the subject of an article published in Nature, is surprising. This galaxy, which has a size close to our galaxy, contains about 200 times less stars and does not appear to contain dark matter.

Astronomers measured the rotation speed of ten groups of stars in this galaxy, concluding that this corresponds to the gravitational pull of visible matter - so there is no need for "extra matter" (ie, dark matter).



It is the first time a galaxy of this kind has been discovered and puts many questions to scientists. How is a galaxy formed? Until recently, it was thought that a kind of skeleton of the galaxy of dark matter originated; The visible part of the galaxy (stars, planets) is then built around it. However, it seems that there is no need for dark matter for a galaxy to emerge. Another hypothesis would be that dark matter existed in this galaxy, but due to violent clashes of cosmic nature, it would have been removed from the galaxy.

The most interesting and bizarre consequence of this discovery, however, is that instead of denying the existence of dark matter, it seems that this galaxy reinforces the evidence that this bizarre form of matter, undetected to date by our instruments, exists in significant quantities in the universe .

How is one possible like this? For calculating the rotation speeds of star clusters in DF2 studied by astronomers, they used the formulas of Newton, demonstrating that they are also valid for enormous distances, such as those in a galaxy. It follows that when the rotation speed of stars in Milky Way Galaxies tells us that matter in the galaxy is not enough based on Newton's formulas, we can trust that result. There is therefore a need for additional matter, probably composed of particles whose mass is unknown. Various models of elemental particle physics predict the existence of particles with extremely small masses or, on the contrary, large - still uncovered, which would make up the mysterious dark matter.

Astronomers now plan to study more galaxies, such as DF2, which contain few stars, just to better understand if they also do not contain dark matter, or DF2 is a strange exception.
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