The galaxies that have no stars

Astronomers using the Very Large Telescope in Chile believe they have made the first ever observations of galaxies that have no stars in them. Writing in the Monthly Notices of the RAS, Sebastiano Cantalupo's team report the discovery of 98 such galaxies, clustered around a bright quasar at a distance of 11 billion lightyears away from the Earth.

These so-called "dark galaxies" fill a gap in theoretical models of how galaxies form, as Martin Haehnelt explained when I spoke to him this week in his office at the Kavli Institute for Cosmology in Cambridge: "We have a standard model of how galaxies form, and there are predictions that we are very confident about that there should be galaxies that are much smaller than are easily visible."

The problem is that the theoretical models predict that stars form so efficiently in the early Universe that galaxies quickly run out of gas to make new stars.

By contrast, there is strong observational evidence that stars formed in the Universe over a prolonged period of 5-10 billion years after the Big Bang, and still continues now, 13.8 billion years later, albeit at a much reduced rate.

Dark, starless, galaxies solve this problem by acting as reservoirs of gas which eventually collide with larger galaxies, providing fresh new gas for later star formation.

"I wouldn't call them failed galaxies", says Haehnelt. "They're slowly developing galaxies and they're the infant stage of bigger galaxies. They will be incorporated later into more normal galaxies."

The difficulty has been working out how to detect galaxies which don't contain any stars. Astronomers usually rely on the radiation from the hot surfaces of stars to light galaxies up. Dark galaxies, by definition, aren't illuminated by any starlight.

"We were looking for another light source... to light them up like a torch", explained Haehnelt. The light source the team chose was a distant quasar, one of the most luminous known objects in the Universe.

It was also at around the right distance, 11 billion lightyears away, to probe the Universe at a time when dark galaxies were thought to be very numerous, 2-3 billion years after the Big Bang.

What the team observed in their 20-hour observation with the Very Large Telescope was the very faint fluorescence of 98 clouds of hydrogen gas, which were being excited by the intense ultraviolet light of the quasar, but which otherwise produced no detectable optical starlight.

For the first time, these dark galaxies are not just a theoretical prediction, but objects that can be observed and measured to test the ideas that cosmologists have about how galaxies have formed and evolved over cosmic history.

The 98 objects seen thus far are all in the same neighbourhood around a single quasar, but given the success of this first observing run, Haehnelt tells me that he's very hopeful of repeating the experiment around other quasars to see how other neighbourhoods compare.