The way dwarf galaxies move puts a new spin on galaxy formation

Small galaxies are playing a game of ring around the rosie. Dwarf galaxies have been caught following each other around the distant galaxy Centaurus A in a coordinated loop, rather than zipping around randomly as theory predicts they should.

The discovery could spell trouble for standard theories of cosmology, including the role of enigmatic dark matter in galaxy formation, astronomers say in the Feb. 2 Science.

These observations suggest “we are missing something, some important piece” in understanding dark matter and how galaxies form, says coauthor Oliver Müller, an astronomer at the University of Basel in Switzerland.

Simulations of how large-scale cosmic structures form suggest that galaxies are connected by a vast network of dark matter, the evasive substance that makes up most of the universe’s matter but interacts with regular matter only via gravity (SN Online: 10/11/17). The largest galaxies grow where threads of dark matter intersect, according to those simulations.

Smaller, dwarf galaxies travel toward the intersections along long dark matter filaments, like cars on roads leading into a city. In the simulations, these dark matter thoroughfares connect major galactic hubs from all directions, so small galaxies should zip around the central galaxy at random.

But they don’t — at least not according to what’s been observed in the local universe. Studies show that both the Milky Way and the nearby Andromeda Galaxy host satellites that orbit in the same plane and the same direction, similar to how the planets in the solar system orbit the sun.

“The Milky Way and Andromeda are good systems to show that there is something weird going on,” Müller says. Scientists had wondered if the effect was limited to these two galaxies, which are both in the same galactic family called the Local Group.

Centaurus A, located about 12 million light-years from Earth, is now the first galaxy system observed outside the Local Group whose satellites move in a similarly coordinated dance. “Our work now shows that other groups have this weird phenomenon going on,” Müller says.

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The satellites of Centaurus A are arranged in at least one flat disk that can be seen edge-on from Earth’s perspective. That view, initially observed in 2015, is “cosmic luck,” Müller says. It allows astronomers to measure the dwarf galaxies’ speeds toward and away from Earth. If the satellites are truly orbiting Centaurus A, not just lined up coincidentally, then some should be moving toward Earth and some away from Earth.

That’s exactly what Müller and his colleagues found. The team analyzed images of Centaurus A and its satellites from the Hubble Space Telescope and the Parkes Telescope in Australia. These observations confirmed that the galaxies lie in a single, relatively thin plane — about 225,000 light-years thick and 2 million light-years wide.

Measuring the motions of 16 dwarf galaxies revealed that 14 follow a consistent pattern. Most of the galaxies that lie above Centaurus A from Earth’s perspective are moving toward Earth, but the galaxies below are moving away. That suggests the galaxies are orbiting Centaurus A as if stuck to a giant cosmic turntable. Simulations suggest only 0.5 percent of galaxies should be arranged this way, the team found.

Müller’s team next plans to measure speeds for another 15 or so satellite galaxies. If those galaxies don’t follow the pack, the finding could fizzle. But if the finding holds up, it could force astronomers to rethink how dark matter guides cosmic structure formation.

Astronomer Michael Boylan-Kolchin of the University of Texas at Austin, who wrote a commentary about the study for the same issue of Science, doesn’t think it will come to that. There’s enough good evidence for dark matter’s effect on the universe that “it would be very premature to say this makes us abandon it,” he says. “But this is also exactly the way we either strengthen the theory, or falsify it.”