Washington DC: According to a recent study, 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 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 - 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, about one billion years after the Big Bang, another important transformation occurred: 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. Using data from an ultraviolet spectrometer aboard the Hubble Space Telescope, the team discovered a nearby compact dwarf galaxy emitting a large number of ionizing photons into the intergalactic medium, or the space between galaxies.
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. From this survey, they identified approximately 5,000 galaxies that match their criteria: 'very compact galaxies emitting very intense UV radiation'. Researchers selected five galaxies for observation with the Hubble Space Telescope.