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AGC 114905

Mystery of ‘non-dark’ galaxies – galaxies with almost no dark matter that cannot be explained by current models deepens

Actually impossible: astronomers have rediscovered a galaxy that doesn’t seem to contain any dark matter even after closer measurements. Its mass and rotation can only be explained by the “normal” matter of gas and stars. However, this contradicts current models of galaxy formation and the cosmic model of cold dark matter, the team explains. Why these “non-dark” galaxies still exist and how they appeared is still a mystery.

According to the popular theory, all galaxies are held together by invisible “dark matter” – dark matter. Its gravity shapes the rotation of galaxies and determines their structure and mass. However, in recent years, astronomers have done some galaxies I discovered that it does not fit into the current picture. because this Galaxies are highly scattered Not only are they very poor in stars for their size, they also seem to have the usual proportion of dark matter. to become absentAs suggested by multiple observations.

contradictory theory

But how could that be? According to current models, dark matter is necessary for galaxies to arise and stick together. “According to our current idea of ​​galaxy formation, we expect each galaxy to be embedded in a halo of cold dark matter,” explain Pavel Mancera Piña of the University of Groningen and colleagues. In the Milky Way, the effect of dark matter is even on motion from their central bar readable.

“Non-dark” galaxies contradict all this – which is why, among other things, there are always doubts about whether these galaxies really contain very little or no dark matter at all. This was also the case for the six super-solid galaxies that Peña and his team discovered in 2019. “You’d better measure again, and then you’ll see that there’s dark matter around your galaxy,” it was said at the time.

Final measurement in Galaxie AGC 114905

That’s exactly what Peña and his team have now done: They’ve targeted New Mexico’s Very Large Array radio antennas in one of six galaxies AGC 114905, 250 million light-years away. From data from 40 hours of observation, the researchers determined the speed of rotation of the galaxy at different speeds, distances from the center and mass of the galaxy. From previous measurements, it was already known that AGC 114905 is about the size of the Milky Way, but contains about a thousand times fewer stars.

Assessments confirmed previous, less accurate measurements: the diffuse galaxy AGC 114905 contains unusually little dark matter and does not appear to have a dominant halo. Instead, the galaxy’s rotation speed, even in its outer regions, can be explained almost entirely by the mass of visible matter in the form of gas and stars, astronomers report.

There is still a contradiction – also in the cosmic model

“This is what we were hoping and expecting,” Peña says. “But the problem remains that theory says there must be dark matter in AGC 114905, but our observations show the opposite address.” This was also confirmed by additional simulations in which the team attempted to model their galaxy on a Cosmological Reproduced Cold Dark Matter (CDM) model.

The result: According to current CDM cosmology, galaxies such as AGC 114905 should not exist. “Even a galaxy with properties of AGC 114905 is practically impossible in the CDM world,” the astronomers wrote. “In order to reproduce the velocity profile, we have to introduce extreme parameter values ​​far beyond the usual frame.” The fact that the team has discovered five other galaxies of this type, albeit slightly less extreme, makes it even more unlikely.

It does not fit any of the common interpretations

But how can these galaxies be explained? As for some other “non-dark” dwarf galaxies, astronomers believe dark matter may have separated from them in a collision or close encounter with another galaxy. These gravitational interactions can mainly occur in dense galaxy clusters. However, in the case of AGC 114905, there is no evidence that this isolated galaxy was once part of such a cluster, Peña and colleagues explain.

Interactions with globular star clusters or encounters of very distant galaxies cannot be confirmed based on the cosmic environment of AGC 114905. Even an alternative model of dark matter, Modified Newtonian Dynamics (MOON), cannot reproduce the behavior of galactic rotation in the model’s calculations.

Another possible explanation could be that astronomers incorrectly measured the angle at which the galaxy stands. However, in order to reconcile the measured spin values ​​and masses with the CDM or MOND model, it would be necessary to perform a measurement deviation of about 20°, Peña and his team determined. They explain that “this discrepancy is six to seven times higher than the nominal uncertainty of our measurements”. Therefore, a measurement error is highly unlikely.

puzzle remains

According to the scientists, their study shows that galaxies that do not contain dark matter or contain very little dark matter actually exist. You’re already measuring the other ultra-diffuse galaxies in your previous study again with a much better VLA resolution. So far, the discovery of many very similar galaxies indicates that they are real and are not based on mere errors of measurement.

“The discrepancy between theory and observation has only increased,” Peña says. “The origin and evolution of these galaxies remains a mystery.” (Monthly notifications of the Royal Astronomical Society, accepted; arXiv: 2112.00017)

Cowell: Netherlands Institute for Radio Astronomy, Royal Astronomical Society