Light from tiny galaxies over 13 billion years ago played a larger role than previously thought in creating the conditions in the universe as we know it today, a new study has found. Ultraviolet (UV) light from stars in these faint dwarf galaxies helped strip interstellar hydrogen of electrons in a process called re-ionization.
The epoch of re-ionization began about 200 million years after the Big Bang and astrophysicists agree that it took about 800 million more for the entire universe to become re-ionized. It marked the last major phase transition of gas in the universe, and it remains ionized today.
Astrophysicists aren’t in agreement when it comes to determining which type of galaxies played major roles in this epoch. Most have focused on large galaxies. However, a new theory by researchers at the Georgia Institute of Technology and the San Diego Supercomputer Center indicates scientists should also focus on the smallest. The findings are reported in a paper published today in the journal Monthly Notices of the Royal Astronomical Society.
The researchers used computer simulations to demonstrate the faintest and smallest galaxies in the early universe were essential. These tiny galaxies – despite being 1000 times smaller in mass and 30 times smaller in size than the Milky Way – contributed nearly 30 percent of the UV light during this process.
Re-ionization experts often ignored these dwarf galaxies because they didn’t think they formed stars. It is assumed that UV light from nearby galaxies was too strong and suppressed these tiny neighbors.
“It turns out they did form stars, usually in one burst, around 500 million years after the Big Bang,” said John Wise, a Georgia Tech assistant professor in the School of Physics who led the study. “The galaxies were small, but so plentiful that they contributed a significant fraction of UV light in the re-ionization process.”
The team’s simulations modeled the flow of UV stellar light through the gas within galaxies as they formed. They found that the fraction of ionizing photons escaping into intergalactic space was 50 percent in small (more than 10 million solar masses) halos. It was only 5 percent in larger halos (300 million solar masses). This elevated fraction, combined with their high abundance, is exactly the reason why the faintest galaxies play an integral role during re-ionization.
“It’s very hard for UV light to escape galaxies because of the dense gas that fills them,” said Wise. “In small galaxies, there’s less gas between stars, making it easier for UV light to escape because it isn’t absorbed as quickly. Plus, supernova explosions can open up channels more easily in these tiny galaxies in which UV light can escape.”
The team’s simulation results provide a gradual timeline that tracks the progress of re-ionization over hundreds of millions of years. About 300 million years after the Big Bang, the universe was 20 percent ionized. It was 50 percent at 550 million years. The universe was fully ionized at 860 million years after its creation.