The first thing that comes to mind when we think of a galaxy are its multiple and bright stars, but there are galaxies in the early universe with very few stars or even none of them. They are known as dark galaxies and now, an international team of astronomers led from the Federal Polytechnic School in Zurich (Switzerland) has identified the signal of six galaxies of this type: just before they begin to form their first stars.
It is believed that these dark galaxies could represent a very primitive stage of galactic formation. According to some theoretical models, they would have been more common in the early universe, when galaxies might have had more difficulty forming stars. However, because dark galaxies do not contain stars, only matter and gas, they emit little visible light, which makes them very difficult to detect and study. The discovery of six new candidates could help us unravel what the dark galaxies really are and what their place is in the formation of galaxies in general.
How did they do it?
Thanks to a combination of a traditional technique and a new technology. The resulting method is based on the presence of quasars, which are some of the brightest objects in the universe, driven by supermassive black holes at the centers of galaxies. The light does not come from the black hole itself, but from the incredible friction in the accretion disk around the black hole. They emit intense ultraviolet light, which radiates fluorescence in nearby hydrogen atoms. This emission is known as the Lyman-alpha (Ly-α) line.
If a dark galaxy, filled with hydrogen, is near a galaxy with a quasar in the center, it will act as a kind of cosmic lantern, and this line will appear in its spectrum.
This technique has been used before, for example in 2012 when it was used to identify candidates for dark galaxies in the early universe using the Very Large Telescope of the European Southern Observatory.
But in 2014, a new instrument was added to the telescope, the Multi Unit Spectroscopic Explorer or MUSE, which allowed the team to look further than previous instruments had allowed, identifying dark galaxies earlier than those previously detected.
The researchers targeted six quasar fields, studying each of them for 10 hours of observation.
Dark galaxies are very difficult to detect and we have only identified a few candidates