This is a Hubble Space Telescope image of the compact green pea galaxy J0925+1403. The diameter of the galaxy is approximately 6,000 light years, and it is about twenty times smaller than the Milky Way. Credit: NASA
Astronomers gain a new understanding of the re-ionization of the universe by studying a nearby dwarf ‘green pea’ galaxy. Newly formed dwarf galaxies were likely the reason that the universe heated up about 13 billion years ago. The finding opens an avenue for better understanding the early period of the universe’s 14 billion year history.
In the period of several hundred thousand yrs after the Big Bang, the universe was so hot and dense that matter was ionized instead of being in a neutral form. But 380,000 years later, the expansion of the universe had cooled it enough for matter to become neutral and for the first structures of the universe to form – gas clouds of hydrogen and helium. Gravity then made these gas clouds grow in mass and collapse to form the first stars and galaxies. Then, ~1 billion yrs after the Big Bang, the universe reheated, and hydrogen – the most abundant element – became ionized for a second time, as it had shortly after the Big Bang, an event which astronomers call “cosmic re-ionization.” How this happened is still debated.
Astronomers have long thought that galaxies were responsible for this transformation. An international team of scientists has largely validated that hypothesis. Using data from an UV spectrometer aboard Hubble, the team discovered a nearby compact dwarf galaxy emitting a large number of ionizing photons into the intergalactic medium which may be responsible for the universe’s re-ionization. “This galaxy appears to be an excellent local analog of the numerous dwarf galaxies thought to be responsible for the reionization of the early universe,” Trinh said.
For reionization to occur, galaxies must eject these photons into the intergalactic medium; otherwise, they are easily absorbed by the gas and dust before they can escape. Despite 20 years of intensive searching, no galaxy emitting sufficient ionizing radiation had been found. To solve this problem, the international research team proposed to observe “green pea” galaxies. Discovered in 2007, these galaxies represent a special and rare class in the nearby universe. They appear green to light sensors and are round and compact, like a pea. They are believed to host stellar explosions or winds strong enough to eject ionizing photons.
The team examined data from the Sloan Digital Sky Survey – a database of more than a million galaxies, and identified ~approximately 5,000 galaxies that match their criteria: very compact galaxies emitting very intense UV. Researchers selected 5 galaxies for observation with Hubble. Using Hubble’s UV radiation detecting capabilities, they found “green pea” galaxy J0925+1403, 3B light-yrs from Earth, was “ejecting” ionizing photons, with an intensity never seen before – about an 8% ejection. This fundamental discovery shows that galaxies of this type could explain cosmic reionization.
“As we make additional observations using Hubble, we expect to gain a much better understanding of the way photons are ejected from this type of galaxy, and the specific galaxy types driving cosmic reionization,” Trinh said. “These are crucial observations in the process of stepping back in time to the early universe. They paved the way to future observations with the successor of Hubble, the James Webb Space Telescope, planned for launch in 2018, which is expected to revolutionize the field with updated capabilities for detailing the first galaxies and sources of cosmic re-ionization.” http://www.eurekalert.org/pub_releases/2016-01/uov-gpg011316.php
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