How do you see what was happening in the early Universe, before the first stars ignited, lighting up the inky blackness?
It's a question that has posed serious problems for astronomers who, in the absence of observational data, have been left pretty much in the dark about this vital stage in the evolution of the cosmos.
According to the theory of galaxy formation immense clouds of diffuse gas and dust created in the aftermath of the big bang must have formed in this early, lightless period in the universe's history. Under the influence of gravity this material began to clump together, eventually condensing into the building blocks of the first stars.
But how to check? Well, the answer it seems, is surprisingly simple: shine a light on it.
And that's exactly what scientists from the Institute for Astronomy in Zurich and the University of California Santa Cruz have done. Using the ESO's Very Large Telescope at Paranal in Chile, the team took advantage of the ultraviolet light emitted by a very bright quasar to tease out the reflected signal of a series of dark galaxies nearby, confirming their existence for the first time.
"We searched for the fluorescent glow of the gas from dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar," says Simon Lilly from the Institute for Astronomy in Zurich. "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."
The team detected almost a hundred dark galaxy candidates within a few million light-years of the quasar, HE 0109-3518, which is one of the brightest in the night sky. After carefully excluding all those where the faint glow might be attributed to hidden internal star-formation - rather than the reflected light from the quasar - they were left with just 12.
Discovering these dark, starless, proto-galaxies has already helped to improve our understanding of the early universe. According to Sebastiano Cantalupo, from the University of California Santa Cruz, the mass of the gas present in each is about a billion times that of the Sun - confirming theories about the structure of gas-rich low-mass galaxies during the Universe's dark age. The team's observations also show that the efficiency of star formation in dark galaxies is suppressed by a factor of more than 100 relative to typical star-forming galaxies.
"We've made a crucial step" he says "towards revealing and understanding the obscure early stages of galaxy formation".
The study "Detection of dark galaxies and circum-galactic filaments fluorescently illuminated bya quasar at z=2.4" will appear in Monthly Notices of the Royal Astronomical Society.