In the history of the universe, the reionization period marks a time when the cosmos transformed from being filled with cool neutral hydrogen (which carried no charge) into a universe with ionized hydrogen that had been split into its component electrons and protons. That change made the hydrogen fog of the early universe transparent to ultraviolet light.
The universe was filled with the neutral hydrogen about 300,000 years after the Big Bang, with the reionization period occurring sometime in the 1 billion years that followed, astronomers have said. Scientists suspect that radiation from the first stars and galaxies caused the reionization.
In the new study, Brown and his colleagues found that the same radiation that triggered the reionization of the universe may have also stunted star formation in dwarf galaxies such as those spotted in the new Hubble telescope views.
The team is actually studying Hubble observations of six faint dwarf galaxies, but only completed its analysis of the Hercules, Leo IV and Ursa Major objects. The Hubble observations are follow-up looks at data from the Sloan Digital Sky Survey, which identified about a dozen of the ultra-faint galaxies.
The three galaxies observed in Brown's study are irregular objects that coalesced about 100 million years before reionization began. They are only 2,000 light-years wide, smaller than the dwarf galaxies seen today near the Milky Way. They are all between 330,000 and 490,000 light-years from Earth.
The process of reionization may have stripped the galactic dwarfs of the vital gas required to build more stars and grow into larger galaxies, the researchers said. With little active star formation, such dwarf galaxies could be all but invisible to astronomers trying to understand why so few of the objects have been found, when theories predict that thousands should be visible, they added.