An international team of astronomers has discovered 780 ultra-faint dwarf galaxies in the early universe. These observations allowed the team to make a crucial assessment of the role of small galaxies in making the universe the way it is today.
The team, lead by researchers at University of California Riverside, focused on the period between 7 and 11.5 billion years ago when the universe reached the peak of star formation. Galaxies were forming stars at least 100 times faster than today, and these new-born stars shined brightly in ultraviolet.
The researchers discovered that these dwarf galaxies are responsible for the emission of about 50 percent of all the ultraviolet radiation at the epoch. They are 10 to 100 times fainter than the previously observed galaxies from this epoch, but there’s a lot more of them. And when it comes to star-forming galaxies, quantity seems to be more important than quality.
Ultraviolet light is responsible for the ionization of hydrogen atoms over 12 billion years ago, which gave the universe a very important quality: it made it transparent. Astronomy wouldn’t be as good if it wasn't.
The discovery of these dwarf galaxies was only possible thanks to an effect of general relativity known as gravitational lensing. Matter can bend spacetime in such a way that background objects are magnified. Black holes are usually given as the textbook example of gravitational lenses, but every massive object can do it.
In this research, published in the Astrophysical Journal, the researchers used three clusters of galaxies called Abell 1689, Abell 2744, and MACS J0717, which are renowned for being spectacular gravitational lenses.
The team used the Hubble Space Telescope and the W. M. Keck Observatory in Hawaii to find the faintest objects from this epoch. This allowed them to produce such an accurate and large census of galaxies that the statistical uncertainties about this result are very small.
The researchers are confident that if dwarf galaxies played such an important role during the peak of star-formation, they might have been crucial since the formation of the first galaxies.