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Dwarf galaxies played major role in lighting up the Universe

Paul Sutherland, Feature writer
Jul 7, 2014, 13:25 UTC

Sen—Fresh evidence has emerged to show the important role that dwarf galaxies performed in producing the Universe that we see around us today.

Despite their tiny size, these collections of stars, which are individually around 1,000 times less massive than our own Milky Way, played a major part in lighting up the Cosmos shortly after the Big Bang.

Sen reported last month how Hubble observations have revealed that dwarf galaxies underwent huge bursts of star formation between two billion and six billion years after the Universe came into existence. New research shows that they were busy much earlier in time, too.

Cosmologists believe that the Universe was ionised just after the Big Bang, with ordinary matter consisting of hydrogen with its positively charged protons stripped of their negatively charged electrons.

Eventually, the Universe cooled enough for electrons and protons to combine and form neutral hydrogen, but for a few million years this was an opaque stage that has been dubbed the “dark ages” because no stars yet shone.

A rendering of a simulation that follows the formation of the first galaxies in the Universe from 200 million to 800 million years after the Big Bang. The video shows hot and ionized gas in blue, and cold and neutral gas in red. Credit: John Wise, Matthew Turk, Michael Norman, Tom Abel, Britton Smith

Astronomers cannot observe this early Universe with conventional telescopes. However, there followed an “epoch of re-ionisation” which was completed about one billion years after the Big Bang and which saw newly formed stars and galaxies illuminate space.

This epoch was the last major change to gas in the Universe and it is still ionised today, more than 12 billion years later.

Until now, astronomers have focused on large galaxies, similar to the Milky Way in size, in trying to find what played the most important role in the process. However, a study involving computer simulations suggests that the smallest galaxies were essential to what happened.

Researchers at the Georgia Institute of Technology and the San Diego Supercomputer Center looked at dwarf galaxies that are often ignored by other studies because it was assumed that the influence of nearby larger galaxies was too strong and would have inhibited them from forming stars.

To the scientists’ surprise, the simulations showed that the dwarf galaxies contributed nearly 30 per cent of the ultraviolet (UV) light that drove the re-ionisation process.

Professor John Wise, of the Georgia Institute of Technology, who led the study, said: “It turns out these dwarf galaxies did form stars, usually in one burst, around 500 million years after the Big Bang. The galaxies were small, but so plentiful that they contributed a significant fraction of UV light in the re-ionisation process.”

A journey around a simulated dwarf galaxy 800 million years after the Big Bang. Older stars are coloured yellow and newer stars blue/white. Credit: John Wise, Matthew Turk, Michael Norman, Tom Abel

The team found that the fraction of ionising photons escaping into intergalactic space was 50 per cent in small galaxies (equivalent to more than 10 million solar masses), but only 5 per cent in larger galaxies (300 million solar masses). This elevated fraction, combined with their high abundance, explains why the faintest galaxies play an integral role during re-ionisation.

“It’s very hard for UV light to escape galaxies because of the dense gas that fills them,” said Wise. “In small galaxies, there’s less gas between stars, making it easier for UV light to escape because it isn’t absorbed as quickly. Plus, supernova explosions can open up channels more easily in these tiny galaxies in which UV light can escape.”

Co-author Professor Michael Norman, of the University of California San Diego, said: “That such small galaxies could contribute so much to re-ionisation is a real surprise. Once again, the supercomputer is teaching us something new and unexpected; something that will need to be factored into future studies of re-ionisation.”