CANARY ISLANDS, Spain — A galactic puzzle has a team of researchers stumped. Scientists from the Instituto de Astrofísica de Canarias and University of La Laguna discovered that the galaxy NGC 1277 does not contain dark matter.
In the current standard model of cosmology, massive galaxies contain substantial quantities of dark matter, a type of matter that does not interact in the same way as normal matter. The only evidence for its existence is the strong gravitational pull it exerts on nearby stars and gas, which is observable.
This is the first time that a massive galaxy, with a mass several times that of the Milky Way, does not show evidence for this invisible component of the universe. “This result does not fit in with the currently accepted cosmological models, which include dark matter,” says lead researcher Sebastién Comerón in a media release.
NGC 1277 is considered a prototype “relic galaxy,” meaning it has had no interactions with its neighbors. These types of galaxies are very rare and are considered remnants of giant galaxies that formed in the early days of the 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.”
The team discovered that the mass distribution in NGC 1277 was just the distribution of stars and inferred that within the observed radius there cannot be more than 5 percent dark matter. However, present cosmological models predict that a galaxy with the mass of NGC 1277 should have at least 10 percent of its mass in dark matter, with a maximum of 70 percent.
“This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model,” says Ignacio Trujillo, a researcher at the IAC and ULL who participated in the study.
Researchers suggest there are two possible explanations for NGC 1277’s lack of dark matter.
“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 Anna Ferré-Mateu, a researcher at the IAC and the 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”.
However, researchers say neither explanation is fully satisfactory.
“So the puzzle of how a massive galaxy can form without dark matter remains a puzzle,” says Comerón.
To continue researching this mystery, the team plans to make new observations with WEAVE instrument on William Herschel Telescope (WHT) at Roque de los Muchachos Observatory on Canary Island of La Palma.
If it is confirmed that NGC 1277 does not have dark matter, it would cast strong doubt on alternative models for dark matter. These theories propose that gravity is modified and most gravitational attraction within galaxies is due to slight change in law of gravity 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 that a galaxy without dark matter is a refutation of this type of alternatives to dark matter,” says Trujillo.
The study was published in the journal Astronomy and Astrophysics.
There is nothing baffling here. This is a very dense, compact galaxy with strong Newtonian/Einsteinian gravitational forces acting on the visible parts. It is completely consistent with the MOND alternative to dark matter; MOND predicts that the dynamics of this galaxy should be almost completely dominated by the Newtonian gravity of its detectable matter, and that’s what we see. It is a powerful example in favour of MOND, ironically by displaying no MOND forces.