Dark Galaxies Discovered in Coma Cluster

A bizarre set of galaxies in the Coma Cluster have lost most of their stars (or star-making material), making them especially rich in dark matter.

The Coma Cluster, visible in the evening skies of spring and summer, reveals its jewel box to large backyard telescopes: several thousand galaxies sardine-packed into a space only 20 million light-years across.

But there’s more to the Coma Cluster than meets the eye — or the backyard telescope.

Pieter van Dokkum (Yale University) and colleagues took a unique look at Coma through the Dragonfly Telephoto Array, eight Canon telephoto lenses coupled to CCD cameras. The Dragonfly is designed to find faint, fuzzy blobs, but what its images revealed surprised the team.

On Coma’s outskirts lurk 47 galaxies similar in size to the Milky Way — but with 1,000 times fewer stars. To survive in crowded Coma, these dark galaxies must contain 98% dark matter to hold themselves together, much higher than the fraction in the universe at large (83%).

Near or Far?

The galaxies' size depends on their distance, so to make sure this result wasn’t just a trick of perspective, van Dokkum and colleagues had to make sure these galaxies really belonged to the Coma cluster, more than 300 million light-years away. If they turned out to live nearby, the galaxies' size would be akin to regular ol' dwarf galaxies.

Determining cluster membership was a challenge because the objects are far too faint to study in the usual ways, such as taking a spectrum to determine their distance. Nonetheless, “van Dokkum and his co-authors make quite a convincing case,” says Mark den Brok (University of Utah).

The authors initially expected the galaxies to be distributed randomly, as they would be if they lay in the foreground near the Milky Way. Instead, the galaxies cluster around the center of the image in the cluster’s periphery. The discovery of a serendipitous Hubble image of one of the galaxies strengthened the team’s case, den Brok says, definitively showing that it doesn’t have the traits of a nearby dwarf galaxy.

Starless Galaxies

Somehow these weirdly faint galaxies have lost their stars — or they never had many stars in the first place. Van Dokkum and colleagues suggest that these may be “failed” galaxies, having forfeited most of their star-building gas after hosting a first generation of stars.

Simulations that track the evolution of large-scale structure suggest that even normal galaxies start out with three times more star-building material than they develop into stars. So whatever process works to limit star formation in normal galaxies is working particularly well in these dark matter-rich galaxies.

“Our simulations have shown that one way to limit star formation so drastically is to use the energy stars produce when they blow up as supernovae,” says Greg Stinson (Max Planck Institute for Astronomy, Germany). “It turns out that this disruption leads directly to galaxies that look like the ones van Dokkum is seeing.”

“I was actually very much relieved to see Prof. van Dokkum’s paper,” Stinson adds. Dark matter simulations have been producing galaxies with exactly the size and matter distribution that van Dokkum’s team observed, but such galaxies are naturally difficult to observe.

It’s ironic that dark matter-rich galaxies were discovered in Coma, the birthplace of dark matter theory. Observations of the same cluster in 1933 helped Fritz Zwicky (Caltech) first conceive of the invisible matter that shapes the large-scale structure of the universe. Now ever-deeper observations continue to help astronomers understand dark matter’s role in galaxy evolution.

Reference:
Pieter van Dokkum et al. "Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster." Astrophysical Journal Letters, submitted.

---

Dark matter is one of the universe's great unknowns. Find out what else we don't know in our special publication, Astronomy's 60 Greatest Mysteries.