Mysteriously dim at the edges (Image: ESA/NASA)

The Pinwheel Galaxy is a darker place than we suspected. Other large spiral galaxies, such as the Milky Way, boast star-speckled outer shells called stellar haloes. But the edges of the Pinwheel Galaxy are mysteriously barren, putting a wrinkle in one of the most widely held theories of galaxy growth.

We think galaxies get bigger either via colliding and merging with large neighbours or by snacking on dwarf galaxies that fall into their gravitational grasp. But big collisions tend to mangle galaxies, and spirals take several billion years to settle into their orderly shapes. So we think most of the spirals we see today grew by gobbling up nearby dwarfs.

This process rips the dwarf galaxies apart and, over billions of years, they leave behind a faint halo of orphaned stars that surrounds the larger galaxy. We have seen such haloes around the Milky Way and our closest large neighbour Andromeda, and simulations suggest that they should be common around spirals across the universe.


Starving galaxy

Until now, though, we had not been able to weigh galaxies other than the Milky Way and Andromeda with enough precision to measure their stellar haloes. Using an array of eight telephoto lenses, Pieter van Dokkum at Yale University and his colleagues were able to measure the visible mass distribution in and around the Pinwheel Galaxy, which lies about 21 million light years away.

Their measurements revealed that its outer halo is strangely devoid of stars. This implies that the spiral galaxy has somehow grown large without feeding on many dwarfs, says van Dokkum.

“There are only a few galaxies that have been studied down to this limit, so it’s hard to say how typical this is,” says Kathryn Johnston at Columbia University in New York. But theorists may be in trouble if galaxies without haloes turn out to be common.

“If we find that the Pinwheel is not an exception, there is something fundamentally wrong,” says van Dokkum. The team plans to use the array to study more galaxies and look for their haloes.

“The fact that they’ve developed this scheme for really being able to map the outskirts of galaxies much farther than people have been able to do before is itself a very nifty thing,” says David Weinberg at Ohio State University in Columbus. “And then this finding of a galaxy without a halo is a very interesting first result.”

Journal reference: Astrophysical Journal Letters, DOI: 10.1088/2041-8205/782/2/L24