Not all galaxies are filled with stars. Astronomers have discovered a horde of nearly starless galaxies each about the size of the Milky Way. How they formed is a mystery, and they imply that there are more ways for a galaxy to evolve than previously imagined.
Pieter van Dokkum, an astronomer at Yale University, and colleagues stumbled across 47 galaxies that stopped forming stars long ago. The stars in each galaxy that remain— about 0.1 percent of the number in the Milky Way — are spread throughout a sphere roughly the size of a typical spiral galaxy. A stargazer living in one of these galaxies might see only a few stars at night, says van Dokkum. “You need something unusual to create a galaxy like this.”
The galaxies live in the Coma cluster, a cache of over 1,000 galaxies about 300 million light-years away in the constellation Coma Berenices. Each of the dark galaxies is just a smudge of light found in images that the researchers acquired with the Dragonfly telescope, made up of eight telephoto lenses all pointing at the same patch of sky. The team also found dark galaxies hiding in old pictures from the Canada-France-Hawaii Telescope and the Hubble Space Telescope. The discovery is described in a paper posted online October 30 at arXiv.org.
The galaxies are red, which means they are filled with cool, dim stars. Young galaxies are blue, dominated by light from hot, massive stars. But as those stars die off, the long-lasting red stars remain behind. These dark galaxies are therefore pretty old. “But what pretty old means is difficult to say,” says van Dokkum. The galaxies seem to be least 4 billion years old, but they could also be nearly as old as the universe, which exploded into existence 13.8 billion years ago.
“This is an oddball category of galaxies that have no natural place in our understanding of galaxy formation,” says Chris Impey, an astronomer at the University of Arizona in Tucson who was part of a team that found a few similar galaxies in the 1980s lurking in the nearby Virgo cluster. “It’s a sobering reminder that our knowledge of galaxies is still incomplete.”
Astronomers believe that galaxies start out as dark lumps of matter whose gravity draws in clouds of hydrogen gas. Once the gas becomes dense enough, stars start to form. As the galaxy ages, fresh supplies of gas from deep space or passing smaller galaxies feed the star-building furnaces. As long as hydrogen is available, the galaxy can keep making new stars.
These newly found dark galaxies imply that either stars can form in galaxies that don’t have much gas (not likely, van Dokkum says) or that something happened to the galaxies billions of years ago. Perhaps the cluster influenced them in some way, van Dokkum says, as nearby galaxies stole star-forming ingredients, though that process would tend to reduce the size of the galaxy as well. “The simple answer is: ‘I don’t know,’” he says.
These galaxies could be important laboratories for understanding dark matter — the elusive, exotic substance that binds galaxies and clusters together. To survive the gravitational tugs from the rest of the Coma cluster, these galaxies need to be 98 percent dark matter and just 2 percent the normal atoms that make up stars and planets. Impey says the motions of the stars could allow researchers to map how dark matter is spread out within each galaxy.