An international team of astronomers has identified the farthest known galaxy group to date. Called EGS77, it is composed of three galaxies whose light comes to us from 680 million years after the Big Bang. That’s less than 5 percent of the age of the universe.
Astronomers have seen more distant galaxies but these are the farthest ones to have been detected all emitting specific ultraviolet light. This emission had an important role: it helped the universe to become transparent by ripping the electrons off the hydrogen atoms across the cosmos.
“The young universe was filled with hydrogen atoms, which so attenuate ultraviolet light that they block our view of early galaxies,” principal investigator James Rhoads from NASA’s Goddard Space Flight Center said in a statement. “EGS77 is the first galaxy group caught in the act of clearing out this cosmic fog.”
Rhoads presented the findings at the 235th meeting of the American Astronomical Society in Honolulu.
Observations showed the galaxy group is contributing to the “epoch of reionization”. In the first few hundred million years of the universe, every new star-forming region was shrouded in clouds of neutral hydrogen. The hydrogen atoms would absorb the light from these stars and continue to do so until their electrons are ripped from their nucleus by certain wavelengths of light. This process is called ionization as it causes the hydrogen to become an ion. Given that this era was the second time in the history of the universe that poor hydrogen got its electron stolen, it is known as the reionization.
“Intense light from galaxies can ionize the surrounding hydrogen gas, forming bubbles that allow starlight to travel freely,” added team member Vithal Tilvi, a researcher at Arizona State University in Tempe. “EGS77 has formed a large bubble that allows its light to travel to Earth without much attenuation. Eventually, bubbles like these grew around all galaxies and filled intergalactic space, reionizing the universe and clearing the way for light to travel across the cosmos.”
Though EGS77 is the first group of galaxies confirmed to be responsible for reionization, it won’t be the last. Upcoming instruments such as the James Webb Space Telescope will find more of these groups, and maybe even more members of EGS77.