Nascent dwarf galaxies actively forming stars were most likely the reason why the universe reionized, several hundred million years after the Big Bang. That is the finding of an international team of astronomers, which has detected photons from a compact galaxy that have enough energy to ionize neutral hydrogen. The researchers' discovery improves our understanding of how the first generation of galaxies reionized neutral gas after the cosmic "dark ages".

Most matter in today's universe lies in the vast swathes of diffuse gas that exists between galaxies, known as the intergalactic medium (IGM). Made up mainly of hydrogen, this gas has changed phase through the history of the universe, especially during its infancy. The early universe was so hot and dense that protons and electrons could not combine to form neutral hydrogen, which was instead ionized. But as the universe expanded, it cooled, and some 380,000 years after the Big Bang, neutral atoms began to form and space became transparent to light.

Universal phase shifts

The gas remained neutral for the next few hundred million years – a dark age that is largely invisible to astronomers because the atoms in the gas absorbed most short-wavelength light. This period prevailed until some very dense regions began to collapse, thanks to gravity. These resulting dense structures that formed within the neutral medium ultimately provided the energetic radiation that ionized all of the neutral hydrogen in the universe. This "epoch of reionization" was the universe's final major phase transition, and triggered the clumpy, structured universe that we see today.

While researchers know that the reionization occurred gradually about 400 million years after the Big Bang, we still do not understand what caused it to occur. Current models show that there were simply not enough galaxies at the time to provide the radiation to cause a universal phase transition. Another snag is that photons and radiation have to escape the galaxies to ionize the IGM. However, star-forming galaxies would be so full of neutral hydrogen that they would absorb much of the ionizing photons, meaning that even for a galaxy producing lots of radiation, only a little would escape its grip.

Bright spark in the dark

Although photons from the reionization epoch will never reach today's telescopes, it may be possible to detect similar photons from closer galaxies coming from when the universe was about two billion – not 400 million – years old. But detecting even this radiation is challenging, because it is often masked by light from other galaxies along our line of sight. Also, it is difficult to work out if these closer galaxies differ from early ones.

After 20 years of effort, one such galaxy has now been found by a team led by Trinh Thuan from the University of Virginia in the US. The researchers have made the first observations of a nearby dwarf galaxy emitting lots of ionizing photons into the IGM. Named J0925+1403, it is a "green-pea galaxy" – a newly discovered, rare kind of galaxy that appears green to light sensors and is round and compact, like a pea. These compact galaxies are thought to be actively producing stars, and can host massive stellar explosions or winds strong enough to eject ionizing photons.

Using data from the Sloan Digital Sky Survey, the researchers identified about 5000 compact galaxies emitting very intense UV radiation, and ultimately selected five galaxies for further observation. Using an ultraviolet spectrometer aboard the Hubble Space Telescope (HST), the researchers studied J0925+1403 and found it to be an ideal candidate – the galaxy's emission lines suggested that the gas around it was unusually highly ionized.

Perfect pea

"This galaxy appears to be an excellent local analogue of the numerous dwarf galaxies thought to be responsible for the reionization of the early universe," says Thuan. The team found that J0925+1403, which lies about three billion light-years from Earth, is ejecting ionizing photons with an intensity never seen before – about 8%, compared with the 1–3% usually seen escaping other nearby galaxies. The total radiation escaping the galaxy is enough to ionize a mass of IGM gas that is almost 40 times greater than the galaxy's stellar mass itself.

The team's discovery suggests that such dwarf galaxies could have played a key role in cosmic reionization. However, more work needs to be done to fully understand what happened in the early universe. The current study looked at just one galaxy, whereas a large population of them would be needed to kick off the reionization epoch.

Same as before?

Writing for Nature News and Views, astronomer Dawn Erb from the University of Wisconsin Milwaukee in the US, who was not involved in the current work, points out that it is also unknown "whether this galaxy is similar to those that reionized the universe; its small size, high-ionization state and relatively low degree of enrichment by elements heavier than helium generally match the expected properties of such objects, but none of these properties have been measured for the earliest galaxies". She adds that the team's observation "emphasizes the need for additional, larger studies" to further our understanding of ionizing radiation in nearby galaxies today, as well as during earlier epochs.

However, Thuan is optimistic about progress. "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," he says. Thuan adds that these observations are the precursors for even better ones that can be made once the James Webb Space Telescope is launched in 2018.

The work is published in Nature.