Early in the Universe's history, something ionized most of the diffuse hydrogen gas that’s spread between galaxies. But until now, the source responsible for this ionization has been largely mysterious—a conundrum so persistent, the authors of a new paper call it “one of the key questions in observational cosmology.”

First, a little background: The Universe’s hydrogen started out ionized because the early Universe was too hot and energetic for electrons to settle down and pair with protons. This situation persisted for about 375,000 years after the Big Bang, at which time the Universe had cooled enough for neutral hydrogen to exist. Then, any light produced by interactions among these hydrogen atoms was at a wavelength where it was quickly re-absorbed by other hydrogen atoms—the Universe was opaque.

It wasn’t until a few hundred million years later that some of the hydrogen in the intergalactic medium (IGM) began to be ionized again by an unknown source of energy. This event is known as the epoch of reionization, and it’s the last major phase transition in the history of the Universe. It returned the Universe to a state where light could travel long distances.

What we know about the process so far is that it occurred as the result of the production of light with wavelengths that were short enough—and thus had enough energy—to cause the ionization. This was a gradual process, but existing models have trouble accounting for the amount of light necessary to reionize the entire IGM within the time it happened. The population of galaxies that existed then, which are thought to be the most likely source, don’t seem to produce enough light.

There’s been no way to directly observe the process in the act, as most of the light in the necessary range of ionizing wavelengths (known as the Lyman Continuum) can never be seen on Earth because it has already been absorbed by the IGM hydrogen that it ionized. Thus far, there’s been no conclusive way to resolve the issue.

The great light escape

To try to understand reionization, researchers have turned their eyes toward relatively nearby galaxies. These have nothing to do with the reionization, but they could be the next best thing. It’s hoped that processes going on in these galaxies (galaxies seen from about 1.6 to two billion years after the Big Bang) are similar enough to those going on in the Epoch of Reionization.

Part of the reason it’s been hard to explain the reionization is that it’s hard for Lyman Continuum light to escape from the galaxies it originates in. Lyman Continuum light has trouble escaping for the same reason it’s able to ionize hydrogen—it’s absorbed by neutral hydrogen. And there’s plenty of neutral hydrogen in the gas in and around most galaxies, so much of it will be absorbed before it can escape to ionize the IGM.

Still, galaxies are likely to be the only significant source of light in that era, so the light must have escaped somehow. Studying these “modern” galaxies could shed some light, so to speak, on how so much light was able to escape those primordial galaxies.

Best available galaxy



The compact dwarf galaxy J0925 + 1403 has properties that make it a good candidate to produce the necessary ionizing radiation. Some of the gas near the galaxy’s star forming regions is extremely ionized (inferred from lines in the galaxy’s spectrum), enough that there’s a good chance the stars are producing more light than the galaxy’s gas can absorb. If so, a significant fraction may be escaping.

A group of researchers set their sights on J0925 + 1403 to see if they could detect ionizing radiation coming from the galaxy. And they did, making it the fourth successful detection of ionizing light from such a nearby galaxy.

This one was different than its predecessors, however. A full eight percent of its ionizing light is escaping rather than the one to three percent seen in the previous galaxies we've examined. This is enough to ionize quite a bit of IGM gas (specifically about 40 times the mass of all the stars in J0925 + 1403), maybe even enough to account for the amount of ionization we observe in the IGM.

Of course, this assumes the early Universe was filled with a population of galaxies like J0925 + 1403, compact dwarfs with a lot of star formation going on (a class of galaxies the researchers dub "green peas"). Some of its properties do match our expectations for what primordial galaxies looked like, though we currently have no direct confirmation.

As is so often the case, further work is required. The authors conclude that larger, statistical studies should help determine whether the case of the ionized IGM gas has been solved.

Nature, 2015. DOI: doi:10.1038/nature16456 (About DOIs)