Magazine article Science News

Some Galaxies Are Short on Stars: Faint Spheres Could Aid Understanding of Dark Matter

Magazine article Science News

Some Galaxies Are Short on Stars: Faint Spheres Could Aid Understanding of Dark Matter

Article excerpt

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 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 more than 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's 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 at 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. …

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