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A Galaxy Without Dark Matter Presents Head-Scratching Cosmic Anomaly
Galaxy NGC 1277 has a surprising lack of dark matter, puzzling scientists.
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Anyone who has ever missed a step running down the stairs and felt the conspicuous absence of the ground knows that there’s a special terror which wells up in the gut when you expect an object to be somewhere, and it isn’t. Astronomers experienced that unusual horror when they recently observed the distant galaxy NGC 1277 and found it entirely devoid of dark matter. The stuff has, apparently, vanished into the particularly thin air of intergalactic space.
Dark Matter, Unexpected and Unfound
The first inkling of the existence of dark matter was uncovered by Fritz Zwicky in 1933. He was using the Mount Wilson Observatory to measure the mass of a galactic cluster when he noticed something weird. Zwicky crunched the numbers on every star, every illuminated piece of matter he could see in the cluster and found their gravity lacking. With so little gravitational force binding them together, Zwicky realized that they should have drifted apart, shattering the cluster, unless there was some other invisible thing holding them together. He called that thing dark matter.
Since then, astronomers have found multiple lines of evidence for the existence of dark matter spread all over the cosmos. It’s so abundant, in fact, that astronomers estimate there’s roughly five times as much dark matter in the universe as regular matter. We know it must be there because of its gravitational influence, but it has otherwise eluded our grasp, a seemingly invisible constituent of our universe.
The discovery of dark matter happened because Zwicky looked to the skies and saw something he didn’t expect to see. In a paper recently published in the journal Astronomy & Astrophysics, an international team of scientists report the opposite problem: a galaxy which appears to have no dark matter at all.
What’s the Deal with the Galaxy NGC 1277?
According to the Standard Model, any galaxy above a certain mass threshold should have a considerable portion of dark matter floating around in its vicinity. The galaxy NGC 1277, located roughly 240 million light-years from here, has several times the mass of the Milky Way and should be comprised of between 10% and 70% dark matter. Astronomers estimate that its actual dark matter is less than 5%, and that's just an error bar. Observations are consistent with zero dark matter.
NGC 1277 is categorized as a relic galaxy, meaning it hasn’t had any interactions with its neighbors in quite some time. As a result, it provides opportunities for astronomers to observe largely unsullied remnants from 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. 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,” lead author Sebastién Comerón said in a statement.
When the team crunched the gravitational numbers on NGC 1277, they found that the mass distribution matched the distribution of visible stars. They didn’t see any evidence of dark matter mass distributed throughout the galaxy. Consequently, they concluded that if NGC 1277 has any dark matter at all, it must be less than 5% of the total.
There are two proposed explanations for the surprising lack of dark matter inside NGC 1277. One possibility is that gravitational interactions with the medium inside the galaxy cluster stripped historical dark matter away. The other possibility is that the merger of protogalactic fragments expelled dark matter, leaving the galaxy particularly vacant.
The team has additional observations planned, using the WEAVE instrument on the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory. Maybe with a little more evidence we’ll be able to solve the mystery of the missing dark matter once and for all.
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