Three years ago, Filippo Fraternali and his colleagues discovered half a dozen mysteriously scattered galaxies, which looked like sprawling cities of stars and gas. But unlike nearly all other galaxies—including our own Milky Way—it doesn’t appear to be entrapped in huge clumps of dark matter, which normally combines these stellar capitals with their gravity. Scientists chose one to zoom in on, a modest-sized galaxy about 250,000 light-years away, and pointed at it 27 radio telescope antennas from the Very Large Array in New Mexico.
After collecting 40 hours of data, they mapped stars and gases and confirmed what the earlier snapshots hinted at: “The dark matter content we infer in this galaxy is much smaller than you’d expect,” Fraternali, an astronomer at the Kapteyn Astronomical Institute at the University of Groningen in the Netherlands. If the team or their competitors find other such galaxies, it could challenge scientists’ view of dark matter, the dominant perspective in the field for at least 20 years. Fraternali and his team published their findings in December in Monthly Notices of the Royal Astronomical Society.
Based on decades of telescope observations and computer simulations, scientists have come to think of dark matter as the hidden skeleton of the universe. Its “joints” are huge clumps of invisible particles that host galaxies large and small. But Fraternali isn’t the first to hint at an exception to this rule. A few years ago, Yale University astronomer Peter van Dokkum and his colleagues discovered similar galaxies using the Hubble telescope that also appeared to be lacking in dark matter. “These galaxies that we found in 2018, have generated a lot of controversy, discussion and follow-up work because they were unexpected and difficult to explain,” van Dokkum says.
Those other galaxies lived in a crowded environment, where neighboring larger galaxies frequently fly in, possibly attracting dark matter with them. By contrast, the Fraternali Galaxy is completely isolated, and has no troublesome neighbors, so the scarcity of dark matter cannot be explained in this way. “They can be very important,” says van Dokkum. How do the stars and gas at that location gather together without the help of dark matter?
These strange objects have come to be called “super-spreading galaxies”. They are very extreme values: in terms of their mass, they are very small, but they are scattered over vast distances. Some are the size of the Milky Way, but with a hundred or fewer stars. They are so close to transparent that it is difficult to spy in the night sky. “It’s fainter in the middle, so it’s harder to detect. Thanks to better telescopes and deeper observations, it’s becoming more and more popular,” says Miria Montes, an astronomer at the Space Telescope Science Institute in Baltimore and an expert on such galaxies.
Beginning in the 1960s, American astronomer Vera Rubin and others first revealed the possibility of invisible or “dark” matter while measuring how fast stars in galaxies rotate around the center, showing that inner stars rotate at different speeds than outer ones. Based on the rotation of those stars, the scientists calculated how much mass the galaxy would have to have to keep it constantly spinning, rather than being slammed into space. For many galaxies, that mass was several times greater than the mass of all the stars combined. Scientists solved the problem by deducing the existence of some kind of dark matter, which does not emit or reflect light, and which should make up the rest of the mass that holds the galaxy together.