Astronomers have found six possible "dark galaxies" – galaxies that instead of being filled with an abundance of stars, don't seem to have many, if any, at all.
According to the most recent astrophysical models, early galaxies might have undergone a dark phase. These dark galaxies, already quite big and full of gas, may have had trouble forming stars. Finding these galaxies is extremely challenging since they don't emit light but by using the light from other nearby sources researchers believe they have spotted six of them.
As reported in the Astrophysical Journal, European researchers have identified these six candidate objects that existed when the universe was not even 2 billion years old. The objects were fluorescently illuminated by nearby quasars, active galaxies powered by the accretion of material falling into a supermassive black hole. This process makes quasars terrifyingly bright, but conveniently so, that their light can be used to look at other objects like a flashlight.
Quasars emit a huge amount of ultraviolet light, which is then absorbed by the gas and re-emitted. In a way, it’s similar to how white clothing shines under a black (ultraviolet) light in a club. And these observations are enough for astronomers to work out a lot of properties of these galaxies.
The six dark galaxy candidates are small, compact objects estimated to have a mass between 200 million and 6 billion times that of our Sun. According to the researchers they have properties similar to other dark galaxies that have been discovered in the last few years, which is what makes them good candidates. The previously discovered ones are more recent than the new suspects though, being in place 3 billion years after the Big Bang, so a full billion later than these newly discovered objects. Discovering earlier dark galaxies can hopefully shed light on the early universe and the formation of galaxies.
The team took advantage of one of the most advanced astronomical instruments available, the Multi Unit Spectroscopic Explorer (MUSE). MUSE is part of the Very Large Telescope’s suite and is capable of observing a wide stellar field in one go, which is why it is extremely well suited to discovering potential dark galaxies neighboring quasars.
Despite the exciting new observations, dark galaxies remain a complex class of objects within the menagerie of cosmic evolution. The few billion years of the universe still lacks wide and in-depth observations due to the limitation of our current instruments. New telescopes, such as the James Webb Space Telescope, and dedicated surveys will help fill in the gaps, and maybe soon these dark galaxies won’t be as mysterious.