Our understanding of the universe is incomplete. The best model we have requires the existence of two substances we have yet to prove to exist: dark energy and dark matter. While that is less than ideal, the model has been far better than the alternatives and it has great predictive power. But not knowing how they behave can lead us to discoveries that don’t make sense – for example, a massive galaxy that doesn’t seem to have dark matter.
The galaxy is called NGC 1277 and it is located 240 million light-years away. It is a massive elliptical galaxy, and a galaxy of its size should have between 10 and 70 percent of its mass as dark matter. Its stellar mass is about 150 billion times the mass of the Sun, that’s not a small galaxy. However, that is not what observations seem to indicate.
Within the radius they observe, astronomers estimate that the galaxy cannot have more than 5 percent of its mass as dark matter and might as well have none. This doesn’t make sense, especially since it is a relic galaxy: a galaxy that has not changed much over the last 12 billion years.
“This result does not fit in with the currently accepted cosmological models, which include dark matter,” lead author Sebastién Comerón, from the Instituto de Astrofísica de Canarias and the University of La Laguna, said in a statement.
“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. 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 lack of dark matter within this distance from the center of the galaxy doesn’t make sense. The scientists have put forward two possible explanations, but neither of them is satisfactory to completely explain what the team is seeing.
“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,” commented co-author Anna Ferré-Mateu, a researcher also 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.”
The lack of dark matter certainly is a problem for the standard model of cosmology and its expectations. But it is even a bigger problem for the alternative theories that get rid of the need for dark matter by changing how gravity behaves instead.
“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,” co-author Ignacio Trujillo, also from IAC and ULL, explained.
The discovery is reported in the journal Astronomy & Astrophysics.