First galaxy without dark matter discovered

It is classified as an ultra-diffuse dwarf galaxy and is the size of our own Milky Way. It is AGC 114905 and is about 250 million light years away.

The first galaxy with no trace of dark matter has been observed: AGC 114905. An international team of astronomers, led by researchers from the Netherlands, made detailed measurements over the course of forty hours with state-of-the-art telescopes on six galaxies with little or no dark matter. The results will be published in the Monthly Notices of the Royal Astronomical Society by Pavel Mancera Piña's team at the University of Groningen and Astron.

The characteristics of the galaxy with no dark matter

When the team of researchers discovered the six galaxies that appeared to have no or little dark matter, they began measurements that lasted a full 40 hours using the Very Large Array (VLA) in New Mexico in the United States. As the measurements were repeated, the evidence of the absence of dark matter became stronger, particularly on the galaxy AGC 114905 which is about 250 million light years away. AGC 114905 is classified as an ultra-diffuse dwarf galaxy and is about the size of our own Milky Way, but contains a thousand times fewer stars. Until now, however, it has been assumed that all galaxies, and certainly ultra-diffuse dwarf galaxies, can only exist if held together by dark matter.

The Dark Matter-Free Galaxy Search

About AGC 114905, researchers collected data on the rotation of its gas for 40 hours between July and October 2020. Then, they made a graph showing the distance of the gas from the center of the galaxy on the x-axis and the rotation speed of the gas on the y-axis. This is a standard way to reveal the presence of dark matter. However, the graph shows that the gas motions can only be fully explained by normal matter.

"This is, of course, what we thought and hoped for because it confirms our previous measurements," said Pavel Mancera Piña. "But now the problem is that the theory predicts that there must be dark matter in AGC 114905, but our observations say that there is not." In their scientific publication, the researchers list one by one possible explanations for the lack of dark matter but that all need to be verified because they do not lead to a clear conclusion.

For example, one motivation could be that AGC 114905 was deprived of dark matter by large nearby galaxies. "But there aren't any. And in the most renowned galaxy formation framework," explained Mancera Piña, "the so-called cold dark matter model, we would have to introduce extreme parameter values that are well beyond the normal range."

In the meantime, researchers are looking in detail at a second ultra-diffuse dwarf galaxy on which there appears to be no trace of dark matter. Should the results be similar to those of AGC 114905, the hypothesis of the existence of galaxies without "dark matter" would become even stronger. Meanwhile, another study has overturned the theory on the origin of dark matter stating that it could arise from the "ordinary" one. Under observation by the scientists there are also the gravitational waves that could solve one of the mysteries of the Universe.

Stefania Bernardini