When galaxies collide NASA, ESA, S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba)

COLLISION imminent. Astronomers have found the first evidence of a dwarf galaxy smash-up about to happen – a finding that lends weight to two cosmological theories.

Pairs of large and medium-sized galaxies have been seen colliding and merging before. It’s a different story with dwarf galaxies: they often hang out at the fringes of larger galaxies, whose gravity strips them of their stars before a merger can take place.

Now Sabrina Stierwalt at the National Radio Astronomy Observatory in Charlottesville, Virginia, and her colleagues have combed the Sloan Digital Sky Survey and found seven isolated clusters containing nothing but dwarf galaxies. Each group is compact enough that at least one pair in each is ultimately likely to merge (arxiv.org/abs/1701.01731).


Elena D’Onghia at the University of Wisconsin, Madison, had predicted in 2009 that unless gravity is doing something very strange, we should see clusters of dwarf galaxies all on their own, even near our galaxy.

“Based on dark matter theory, we expect a lot of little dwarf galaxies and clumps of dark matter in and around the Milky Way,” she says. So why have these clusters been so hard to spot?

The answer lies in the idea that dwarf galaxies merge to form bigger galaxies like the Milky Way. If so, then few would survive nearby. Looking into the distant, early universe, we would expect dwarf galaxies to be numerous but also too faint to see.

“Dwarf galaxies live inside dark matter haloes, without which they wouldn’t hold together”

Seeing these seven clusters fairly nearby suggests our theories of how galaxies form via collisions are right. It also suggests invisible dark matter is distributed much as we thought – in small patches throughout the universe, says Mike Boylan-Kolchin at the University of Texas, Austin.

Without the gravity of dark matter, dwarf galaxies wouldn’t be able to hold together. “These galaxies are living in dark matter haloes,” says Boylan-Kolchin.

As technology improves, we can hope more of these faint systems will come into view.

This article appeared in print under the headline “Dwarf galaxy merger is doubly pleasing”