Ultraviolet light from a quasar (in the red circle) causes hydrogen gas in dark galaxies (blue circles) to fluoresce.
ESO/Digitized Sky Survey 2/S. Cantalupo (UCSC)
According to our best models, the early Universe was filled with clouds of gas that were the source of its current galaxies. However, not all of these protogalactic nebulas were massive enough to compress their gas sufficiently to make stars. This means a number of "dark galaxies" should exist: galaxy-sized clouds of gas with few or no stars. This lack of stars makes dark galaxies extremely hard to find, but a group of astronomers using the Very Large Telescope (VLT) in Chile have identified 12 candidates from the early days of the Universe.These results have the potential to fill in some of the observational gaps in our understanding of early galaxy formation and evolution.
The trick for spotting these dark galaxies is described in Monthly Notices of the Royal Astronomical Society
by Sebastiano Cantalupo, Simon J. Lilly, and Martin G. Haehnelt, who located a quasar, a supermassive black hole pumping out large quantities of ultraviolet (UV) light. Just as UV emission from mercury drives fluorescent lights, the UV light from the quasar caused gas in the dark galaxies to fluoresce, making them faintly visible.
The researchers estimated the candidate clouds' masses would be comparable to dwarf galaxies today, much smaller than star-forming galaxies at the same distance from Earth. They also identified filaments of gas, which might be the signature of material falling in to the dark galaxies.
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