According to a multinational team of astronomers, led by Dr Yuri Izotov from the Main Astronomical Observatory in Ukraine, so-called ‘green pea’ galaxies were the reason that the Universe heated up about 13 billion years ago.
This Hubble image shows the green pea galaxy J0925+1403. Image credit: NASA.
In the period of several hundred thousand years 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 – clouds of hydrogen and helium. Gravity then made these clouds grow in mass and collapse to form the first stars and galaxies.
Then, about one billion years after the Big Bang, the Universe reheated, and hydrogen became ionized for a second time. How this happened is still debated.
Scientists have long thought that galaxies were responsible for this transformation.
Dr Izotov and his colleagues from the United States, Ukraine, Switzerland, the Czech Republic, France, and Germany, have largely validated that hypothesis in a study published this week in the journal Nature.
Using data from the NASA/ESA Hubble Space Telescope, the astronomers discovered a low-mass star-forming galaxy (named J0925+1403) emitting a large number of ionizing photons into the intergalactic medium. They believe those photons are responsible for the reionization of the Universe.
According to the astronomers, J0925+1403 is a ‘green pea’ galaxy located at a distance of 3 billion light-years away.
An analysis of the galaxy shows that it measures merely 6,000 light-years across, about 20 times smaller than our own Milky Way Galaxy.
Green pea 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.
Using Hubble’s UV radiation detecting capabilities, Dr Izotov and co-authors found that J0925+1403 was ‘ejecting’ ionizing photons, with an intensity never seen before – about an 8 percent ejection. This fundamental discovery shows that galaxies of this type could explain cosmic reionization.
“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,” said team member Prof. Trinh Thuan, from the University of Virginia.
“The finding is significant because it gives us a good place to look for probing the reionization phenomenon, which took place early in the formation of the universe that became the universe we have today.”
He added: “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.”
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Y. I. Izotov et al. 2016. Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy. Nature 529, 178-180; doi: 10.1038/nature16456
