Astronomers have long been questioning how galaxies could have ionized all the gas present in the universe after the Big Bang, leading to the universe we see today, and struggled to match the strength of the emission from galaxies with how quickly reionization happened. But a citizen science discovery has led them to work out the possible culprit for cosmic reionization.
An international group of astronomers believes that green pea galaxies are responsible for reionization. Green peas are small compact objects that can emit very powerful photons capable of turning neutral hydrogen into ionized plasma. Green pea galaxies were discovered in 2007 by volunteer users of Galaxy Zoo, and they are so-named because they resemble peas.
After the Big Bang, the universe experienced a long epoch where no light was shining, aptly called the cosmic dark ages. During this time, the universe was populated by large clouds of neutral hydrogen. This uneventful time for the gas came to an abrupt halt when the birth of stars turned the hydrogen into a plasma, the beginning of the reionization of the universe.
Cosmic reionization is still mostly a mysterious epoch. For neutral hydrogen to be ionized you need to have very powerful UV photons. Young stars emit significantly in UV, but if these stars are buried deep in large galaxies, those photons would be absorbed on the way and never escape the galaxy.
The research was published this week in Nature. Talking to IFLScience, coauthor Anne Verhamme from the University of Geneva said: “Quasars [the super-heated material of dust and gas around a supermassive black hole] are thought to be not numerous enough to have a significant contribution. Young stars in galaxies are considered as the usual suspects among our community of astronomers studying cosmic reionization.”
Although galaxies are considered the culprit, the average galaxy doesn’t emit enough ionizing photons. So the hunt began to look for powerful galaxies. Using the Hubble Space Telescope, the team observed “green pea” galaxy J0925 emitting ionizing photons with extraordinary intensity. The emission is more powerful than most galaxies, and if a large population of these objects was discovered in the early universe, they could certainly explain how reionization happened.
Although the emission is in ultraviolet, due to the expansion of the universe, the light from these galaxies will appear in infrared. To find out if the team is right about the green peas, it will be necessary to wait for the next generation of space telescopes. "We need the James Webb Space Telescope because we need an infrared telescope to detect them,” said Verhamme.
Technical limitations and the simple fact that there were fewer luminous objects in the universe, make reionization difficult to study. This discovery will have a significant impact on the field. The team claims in a statement that “these discoveries represent an important step for studies of the early universe.”