Dark galaxies spotted for first time

Posted by Emma Woollacott

Long-theorized, but never before seen, the dark galaxies of the early universe have now been directly observed by ESO's Very large telescope (VLT).

These small, gas-rich galaxies are very inefficient at forming stars themselves, but are believed to have fed large galaxies with much of the gas that later formed into the stars that exist today.

Because they are essentially devoid of stars, these dark galaxies don't emit much light, making them very hard to detect - although small absorption dips in the spectra of background sources of light have hinted at their existence.

"Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it," says Simon Lilly of ETH Zurich.

"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."

It took some very long exposures, but the VLT was finally able to detect the extremely faint fluorescent glow of the dark galaxies, identifying nearly 100 gaseous objects which lie within a few million light-years of the quasar.

After eliminating objects where the emission might be powered by internal star-formation in the galaxies, rather than the light from the quasar, they  narrowed down their search to 12 objects.

The team was even able to determine some of the properties of the dark galaxies. They reckon the mass of the gas in them is about one billion times that of the sun, and that they're 100 times less efficient at forming stars than most galaxies of the time.

"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," says Sebastiano Cantalupo of the University of California, Santa Cruz.