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A group of astronomers from Canada and the United States has proven the existence of anomalous galaxies that lack dark matter. Previously, it was believed that the presence of dark matter in large galaxies is mandatory. The results also challenge alternative gravity (modified Newtonian mechanics), which deny the existence of dark matter and its influence on the motion of stars in galaxies. The researchers’ article was published in The Astrophysical Journal Letters.

Recent observations have shown that large, low-luminosity (ultra-diffuse) galaxies NGC 1052-DF2 and NGC 1052-DF4, adjacent to each other, contain low amounts of dark matter and few bright globular clusters. To interpret this data correctly, you need to know the exact distance to galaxies. Previously, scientists determined that the distance to NGC 1052-DF2 reaches about 20 megaparsecs (about 60 million light years). To do this, they resorted to the tip of the red giant branch (TRGB) method, which involves the Hubble telescope identifying the brightest red giant stars. Such stars at a certain stage of their existence have the same luminosity, therefore, by their relative stellar magnitude, one can measure the distance to the galaxy in which they are located.

In the new work, astronomers used one of the main instruments of the Hubble Space Telescope – the ACS (Advanced Camera for Surveys) camera – to identify the orbits of 40 red giants in the second galaxy, DF4. By measuring the distance to the galaxy using the TRGB method, scientists have obtained a value of about 22 megaparsecs. This result excludes alternative interpretations of the observation of a small amount of dark matter, which could be correct if the galaxy were much closer (about 13 megaparsec) and would be, respectively, smaller and less bright.

Farther distance means that the part of the galaxy that consists of visible (baryonic) matter is more massive, and the influence of this mass fully explains the motion of stars in DF4. Conversely, in normal galaxies like the Milky Way, additional hidden mass affects the movement of stars, which is evidence of the presence of dark matter. Deprived of dark matter, massive galaxies pose a problem for modern cosmology, according to which clusters of invisible matter played a decisive role in the formation of galaxies and galaxy clusters. At the same time, ideas that reject dark matter and introduce a modified theory of gravity are also under attack. Ultradiffuse galaxies DF2 and DF4 confirm the validity of Newton’s laws.

New data show that the presence of dark matter in galaxies is not necessary at all. It is possible that in the course of their evolution, ultradiffuse galaxies have somehow lost dark matter, for example, as a result of tidal interactions with other galaxies. Astronomers plan to continue searching for ultradiffuse galaxies to understand how they form, how widespread they are, and what properties they have, which in turn will refine the standard cosmological model.

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