For the first time, a massive galaxy has been found to contain no dark matter, puzzling cosmologists.
NGC 1277 is a galaxy several times the mass of the Milky Way. Its bizarre makeup is detailed in the journal Astronomy and Astrophysics.
“This result does not fit in with the currently accepted cosmological models, which include dark matter,” says first author Dr Sébastien Comerón, an astronomer at the University of La Laguna (ULL) in Spain and a researcher at the International Astronomical Center (IAC).
In the current Standard Model of Cosmology, dark matter accounts for about 85% of all the matter in the universe, meaning only one-sixth is made up of the “ordinary” visible matter with which we interact every day.
Dark matter is elusive. It does not interact in the same way as normal matter, the only evidence for its existence is the gravitational effect that it has on the stars and gas in galaxies.
NGC 1277 is a “relic galaxy” – one which doesn’t interact with its neighbours. Relic galaxies are very rare and are believed to be the remnants of giant galaxies which formed in the early universe.
“The importance of relic galaxies in helping us to understand how the first galaxies formed was the reason we decided to observe NGC 1277 with an integral field spectrograph” explains Comerón. “From the spectra we made kinematic maps which enabled us to work out the distribution of mass within the galaxy out to a radius of some 20,000 light years.”
According to their results, NGC 1277 contains no more than five percent dark matter. Its mass distribution was accounted for by its stars alone.
Present cosmological models predict that a galaxy of this size should be 10–70% dark matter.
“This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model,” says Dr Ignacio Trujillo, a researcher at IAC and ULL.
The team suggests two possibilities for the lack of dark matter in NGC 1277.
“One is that the gravitational interaction with the surrounding medium within the galaxy cluster in which this galaxy is situated has stripped out the dark matter,” says Dr Anna Ferré-Mateu, also from IAC and ULL. “The other is that the dark matter was driven out of the system when the galaxy formed by the merging of protogalactic fragments, which gave rise to the relic galaxy.”
But neither theory has satisfied the researchers.
“The puzzle of how a massive galaxy can form without dark matter remains a puzzle,” says Comerón.
They plan to continue observations using the William Herschel Telescope at the Roque de los Muchachos Observatory, on the Canary Island of La Palma.
Perhaps surprisingly, the apparent lack of dark matter in NGC 1277 is further evidence for its existence and casts significant doubt over alternative theories for the observed effects in galaxies, such as those that put forward a slight modification of gravitational laws on large scales.
“Although the dark matter in a specific galaxy can be lost, a modified law of gravity must be universal. It cannot have exceptions. So, a galaxy without dark matter is a refutation of this type of alternative to dark matter,” Trujillo says.
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