Astronomers have discovered another galaxy whose rotation rate is explained by the presence of only one baryonic matter. The measured velocities are consistent with the presence of a small amount of dark matter, but it does not fit into any cosmological model of galaxy formation.
Dark matter theories
The assumption of the existence of dark matter in the Universe was made in the 1930s by the American astrophysicist Fritz Zwicky to explain the discrepancy between his measured speed of motion of galaxies in the Coma cluster and the speed that could be expected based on the total mass of matter in the cluster observed in a telescope. The presence in it of a sufficient amount of previously unknown matter, which was called dark matter, could explain the too high measured speed of galaxies.
This hypothesis was confirmed in the 1970s by observations of the astronomer Vera Rubin, who measured the rotation speeds of stars located on the periphery of the Andromeda galaxy, and found that they are close to the speeds of stars near the center, which, again, was incompatible with the observed mass of matter in the galaxy. As with Zwicky’s observations, the problem was solved by the presence of a large amount of invisible matter, and its mass should have been tens of times greater than the mass of baryonic matter.
Role of dark matter
The existence of dark matter is also confirmed by the peculiarities of the inhomogeneity of the cosmic microwave background and the fact that without this substance, galaxies and their clusters would not have had time to form during the lifetime of the Universe.
Considering the important role that dark matter plays in explaining the dynamics of galaxies and their clusters, the discovery of galaxies that do not contain this type of matter came as a surprise to scientists. The discovered galaxies belong to the type of ultradiffuse dwarf galaxies, with a matter density and luminosity hundreds and thousands of times less than that of the Milky Way.
Later, however, the anomalousness of these galaxies was questioned: it turned out that astronomers had incorrectly measured the distance to one of them, which led to an incorrect estimate of the masses of different types of matter in it, and from the second, dark matter could be pulled out by a nearby massive galaxy. It would seem that these adjustments have solved the problem of the existence of galaxies without dark matter.
Galaxy ACG 114905, which has little or no dark matter
In 2021, a group of astrophysicists from the UK, the Netherlands, and the United States led by Pavel E. Mancera Piña of the University of Groningen and the ASTRON Institute discovered the galaxy AGC 114905, which has little or no dark matter.
This galaxy is located at a distance of 76 ± 5 megaparsecs from the Earth and is a disk in which the density of stars decays as e -r / R when moving away from the center at a distance r, and the characteristic distance scale R is 1.79 ± 0.04 kiloparsecs.
The mass of the galaxy is (1.4 ± 0.3) × 10 9 masses of the sun. The total observation time for the galaxy with the VLA telescope was 40 hours from July to October 2020. Like the previous two candidates for galaxies without dark matter, the ultradiffuse dwarf galaxy AGC 114905 is rich in gas.
Rotation rates of matter in the galaxy
The researchers measured the rotation rates of matter in the galaxy, and it turned out that no dark matter is needed to explain them – there is enough ordinary baryonic matter concentrated in stars and interstellar gas. If there is dark matter in a galaxy, then it is not tens of times more than baryonic matter, as in most galaxies (or at least 5, as in the Universe on average ), but about six times less, which does not fit into modern cosmological models of galaxy formation.
Scientists believe that the long and careful observation of the galaxy AGC 114905, which they conducted, excludes the possibility of a significant error in determining the distance to it. Нamely, this was the reason for the erroneous conclusion about the absence of dark matter in one of the previous cases. Also, in the vicinity of AGC 114905, there is no sufficiently massive companion that could pull dark matter out of it.
Another potential solution to the riddle with an unusually low mass of dark matter in the galaxy is the very large size of the halo with a very low density. However, this, like the already mentioned low density in itself, does not fit into the modern understanding of the process of galaxy formation. To test this hypothesis, it is necessary to measure the velocities of matter in the galaxy at a greater distance from the center.
Sometimes, to explain the absence of observations of dark matter particles in laboratory experiments, scientists use one particular hypothesis. It states that dark matter does not exist, and the contradiction between the observed speed of movement of matter in galaxies and the predictions of the theory is explained by the fact that the theory of gravity must be modified on a galactic scale.
This theory is called Modified Newtonian Dynamics (MOND). However, the predictions of this theory also strongly disagree with observations of AGC 114905.
The researchers cite the incorrect estimate of the galaxy’s tilt angle as the main possible source of error but point out that for the AGC 114905 observational data to be consistent with the existing understanding of cosmology, this error must be incredibly large.
Moreover, five more candidates for galaxies without dark matter were discovered recently, which have a different inclination in relation to the line of observation from the Earth. Scientists believe further astronomical observations are required to confirm the absence of dark matter in some ultradiffuse dwarf galaxies.
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