The first thing that comes to mind when we think of a galaxy are its many bright stars, but there are galaxies in the early universe with very few or even no stars. They are known as dark galaxies and now, an international team of astronomers led from the Federal Polytechnic School of Zurich (Switzerland) has identified the signal of six galaxies of this type: just before their first stars begin to form.
These dark galaxies are believed to represent a very early stage of galactic formation. According to some theoretical models, they would have been more common in the early universe, when galaxies might have had a harder time forming stars. However, because dark galaxies contain no stars, only matter and gas, they emit little visible light, making them very difficult to detect and study. The finding of six new candidates could help us unravel what 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 fluoresces nearby hydrogen atoms. This emission is known as the Lyman-alpha (Ly-α) line.
If a dark, hydrogen-filled galaxy is close to a galaxy with a quasar in the center, it will act as a kind of cosmic lantern, and this line will appear on its spectrum.
This technique has been used before, for example in 2012 when it was used to identify dark galaxy candidates in the early universe using the European Southern Observatory’s Very Large Telescope.
But in 2014, a new instrument was added to the telescope, the Multi Unit Spectroscopic Explorer, or MUSE, allowing the team to look farther 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.
In this way, they obtained complete spectral information for each of the possible dark galaxies, and were able to distinguish around 200 Lyman-alpha emitters, which ruled out that they were usual star-forming galaxies.
There is no strong evidence yet that all six are dark galaxies, but they look much more like this than any other object we know of. Which means that, as predicted in 2012, MUSE could be a powerful tool for hunting down these mysterious objects.
“Each quasar field observed with MUSE will offer the potential to discover new dark galaxy candidates and provide crucial information on the early and dark phases of galaxy formation,” the authors write in The Astrophysical Journal.
Reference: Dark Galaxy Candidates at Redshift ~ 3.5 Detected with MUSE. Raffaella Anna Marino, Sebastiano Cantalupo, Simon J. Lilly, Sofia G. Gallego, Lorrie A. Straka, Elena Borisova, Gabriele Pezzulli, Roland Bacon, Jarle Brinchmann, C. Marcella Carollo. The American Astronomical Society. 2018 The Astrophysical Journal
