Strike a light! Starless galaxies discovered

Astronomers have made the first observations of the earliest dark, starless galaxies by shining a light on them – just as you might view objects with a torch in a darkened room.

Dark galaxies ringed in blue surround the red-ringed quasar. Credit: ESO, Digitized Sky Survey 2 and S. Cantalupo (UCSC)

Of course it is not possible to direct your own flashlight across cosmic distances at will, but the scientists used a natural brilliant source called a quasar to illuminate the gloom.

They turned Europe’s giant Very Large Telescope (VLT) in Chile on one quasar labelled HE0109-3518 in the small southern constellation of Sculptor. This highly luminous object is believed to mark the site of a supermassive black hole and shines due to the incredible energy produced.

The European Southern Observatory’s powerful telescope revealed faint smudges around the quasar that are small, gas-rich galaxies which had been predicted to exist in the early Universe more than 13 billion years ago before stars began to shine.

These dark galaxies – not to be confused with dark matter – are thought to have been the building blocks that contributed material to form larger, star-rich galaxies that we are familiar with today. Previously astronomers had got hints of the dark galaxies’ existence from tiny clues in the spectra of other distant objects, but they had never been directly seen.

The team, led by Sebastiano Cantalupo, of the University of California, Santa Cruz, knew they had to be careful not to confuse dark galaxies lit up by the quasar with galaxies where the glow might be caused by stars beginning to shine. But from nearly 100 blurry objects found within a few million light-years of the quasar, they whittled them down to 12 which they are pretty confident are examples of the long-sought dark galaxies. Their results are reported in the journal Monthly Notices of the Royal Astronomical Society.

Close-ups of the 12 dark galaxies discovered. Credit: ESO, Digitized Sky Survey 2 and S. Cantalupo (UCSC)

The astronomers used the VLT’s FORS2 instrument to map a region of the sky around the quasar, looking for the ultraviolet light that is emitted by hydrogen gas when it is subjected to intense radiation. Team member, Simon Lilly, of the Institute for Astronomy, ETH Zurich, Switzerland, said: “Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it.

“We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a night club.”

Cantalupo said: “After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects. Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we’ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas.”

An instrument called the MUSE integral field spectrograph, which is due to come online on the VLT in 2013, will be a powerful new tool to help discover more about these dark galaxies.

And as we reported last month, a complex new instrument called KMOS has been completed that will peer back more than 13 billion years to see the first stars switch on in the darkness that followed the Big Bang.

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