The enigma of the massive galaxy without dark matter

An international investigation, led by the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL), finds the first evidence of a massive galaxy without dark matter. The result challenges the current standard model of cosmology. The study is published in the specialized journal Astronomy & Astrophysics.

A scientific team, led by the IAC and University of La Laguna (ULL) researcher Sébastien Comerón, has discovered that the galaxy NGC 1277 does not have dark matter. It is the first time that a massive galaxy (with several times the mass of the Milky Way) has been found without there being evidence of this invisible component of the Universe. “This result has no place within the current paradigm of the cosmological model with dark matter”, explains Comerón.

The standard cosmological model postulates that massive galaxies contain large amounts of dark matter, a type of matter that is transparent and does not interact with ordinary matter, but whose existence can be inferred from the gravitational pull it exerts on observable stars and gas.

NGC 1277 is known to be a prototype ‘relic galaxy’, that is, a galaxy that has not interacted with any of its neighbors. These galaxies are extremely rare and are considered unevolved remnants of what were giant galaxies at the dawn of the Universe.

“The importance of relic galaxies in understanding how the first galaxies formed was the reason why we decided to observe NGC 1277 with an integral field spectrograph,” says Comerón. “From these spectra, we obtained kinematic maps with which we reconstructed the mass distribution of the galaxy within a radius of about 20,000 light years,” he adds.

The team has discovered that the distribution of the mass of NGC 1277 corresponds to that of the stars, so it follows that, within the sampled radius, there could be a maximum of 5% dark matter, although the observations support the non-existence of this component.

However, cosmological models predict that a galaxy with the mass of NGC 1277 should have a dark matter fraction of at least 10% and up to 70%. “This discrepancy between the observation and what is expected is an enigma and perhaps a challenge for the standard model”, points out Ignacio Trujillo, a researcher at the IAC and the ULL who has participated in the study.

The study proposes two possible explanations for the lack of dark matter in NGC 1277. “One is that the gravitational interaction with the medium of the cluster of galaxies in which it is found has removed the dark matter,” says Anna Ferré-Mateu, a researcher at the IAC and the ULL who has also participated in the study. another is that dark matter was ejected from the system when it formed through the fusion of protogalactic fragments that gave rise to the relic galaxy.

For the authors of the study, none of these explanations is entirely satisfactory, “with which the enigma of how a massive galaxy can be formed without dark matter remains open,” Comerón emphasizes. In order to further investigate this mystery, the team plans to make new observations with the WEAVE instrument of the William Herschel Telescope (WHT), located at the Roque de los Muchachos Observatory, on La Palma.

If the result that NGC 1277 does not have dark matter is confirmed, the discovery would call into question alternative models of dark matter, that is, modified gravity theories that explain that much of the gravitational attraction between galaxies is due to slightly altered rules of gravity. “Although the dark matter of a galaxy can be lost, a modified law of gravity has to be universal and cannot have exceptions, so a galaxy without dark matter is a refutation of the alternatives to dark matter,” Trujillo emphasizes.