Using a cosmic flashlight to illuminate dark galaxies

Astronomers report possible sightings of gas-filled galaxies that might explain star formation. Andrew Masterson reports.

The Very Large Telescope in Chile, searching for very dark things a very long way away.
The Very Large Telescope in Chile, searching for very dark things a very long way away.
Peter Langer/Getty Images

Astronomers have used the intense ultraviolet light emitted by quasars to potentially identify six mysterious “dark galaxies” – a find that promises to provide answers to one of the most stubbornly baffling questions in the field.

Scientists know that the hot, diffuse X-ray-emitting gas that exists between galaxies, known as the intergalactic medium, must play a role in star formation, but the mechanisms of this involvement remains unknown.

The leading theory suggests that in their infancy, nascent galaxies contain a large amount of the gas, but still fail to form stars. This has been dubbed the hypothetical “dark phase” of galaxy formation, and the results known as “dark galaxies”.

Discovering evidence to support the idea, however, necessarily means finding a galaxy going through a dark phase – a matter made extremely difficult because such galaxies (assuming they exist) would not produce or reflect light, remaining thus invisible to current detection methods.

In a paper published in the Astrophysical Journal, however, a team led by Raffaella Anna Marino and Sebastiano Cantalupo, both from science university ETH Zurich in Switzerland, present the tentative first candidates for observed dark galaxies.

To make their findings the team used the intense ultraviolet light emitted by quasars – which induces fluorescence in a type of hydrogen atoms, causing them to emit a spectral line known as Lyman-alpha – meaning that such atoms become easier to detect, even when they form part of otherwise dark entities.

While not the first researchers to take advantage of this effect, Marino and her colleagues used the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile to look at possible targets at a far greater distance than ever before.

From an initial group of 200 Lyman-alpha emitters, the astronomers eventually narrowed the field to just six – which they say represent “robust” candidates for dark galaxies.

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