In a new study, researchers describe observations of a distant galaxy, far enough away to be seen as it was a mere 700 million years after the Big Bang (as of this writing, it’s been about 13.8 billion years since the Big Bang). These observations turned up both the most distant oxygen ever observed and new clues about the cosmic phase shift known as reionization.

Early in the Universe’s history, there were no stars and little of any elements heavier than hydrogen and helium. When the first stars began to appear, they made two changes in the Universe around them. First, they began to create some of the heavier elements. Second, they started ionizing the hydrogen gas, stripping the electrons off it and leaving it electrically charged.

This second process is called reionization, since hydrogen started out ionized after the Big Bang and only turned neutral after the Universe cooled. We don't fully understand the reionization era of the Universe’s history, making it a major point of study.

One aspect researchers are looking into is the role that the first galaxies played in the process. Galaxies produce stars, which in turn generate enough light that they could ionize the Universe’s gas. But to do that, the light has to first escape from its galaxy, which also contains a lot of gas. This local gas could trap the light within the galaxy. So we need some observations of the early Universe to understand what drove reionization.

Peering into the Dark Ages

If heavy elements such as oxygen were detected in that early epoch, the researchers reasoned, it would paint a picture of what kind of star formation was going on then and thus what kinds of stars existed. Since it's the stars that provide the light for reionization, that's a crucial part of the puzzle.

“Seeking heavy elements in the early Universe is an essential approach to explore the star formation activity in that period,” said Akio Inoue of Osaka Sangyo University, Japan, the new paper's lead author. “Studying heavy elements also gives us a hint to understand how the galaxies were formed and what caused the cosmic reionization.”

The researchers decided to look for the presence of these heavy elements using the Atacama Large Millimeter/submillimeter Array (ALMA). In order to gain access to ALMA for the study, the researchers had to first make the case that the telescope would be able to detect the elements at these distances. So the team performed a computer simulation of the galaxy in question, SXDF-NB1006-2, which showed that if it's there, the oxygen should show up clearly in ALMA data.

“We expected that the light from ionized oxygen is strong enough to be observed, even 13 billion light-years away,” explained Hiroshi Matsuo at the NAOJ, “because the Japanese infrared astronomy satellite AKARI has found that this emission is very bright in the Large Magellanic Cloud, which has an environment similar to the early Universe.”

In the spectrum of light coming from the galaxy, the researchers detected a line that indicates the presence of oxygen—the most distant oxygen ever detected. The researchers estimate that the galaxy has an oxygen abundance about one-tenth that of the Sun, which matches theoretical predictions for galaxies in that era of the Universe’s history.

“The small abundance is expected because the Universe was still young and had a short history of star formation at that time,” explained Naoki Yoshida of the University of Tokyo, another of the paper's authors. “In fact, our simulation predicted an abundance ten times smaller than the Sun."

From the ultraviolet light the galaxy's putting out, the researchers estimated that the "current" period of star formation (current from the galaxy's perspective, during the Dark Ages) had been going on for about a million years. However, that period is not long enough to have produced the oxygen observed, so there must have been earlier episodes of star formation. That revelation provides a lot of information about the stars available to contribute to reionization.

But that's not all. "We have another, unexpected, result: a very small amount of dust,” said Yoshida. The dust in question is made of heavy elements, which were relatively rare (10 percent of present values) at the time. But there was much less dust than heavy elements in the galaxy, indicating that much of the heavier material is in a gaseous form. That situation could be explained by frequent supernovas disrupting dust, or alternatively by a lack of the cold, dense gas clouds that would quickly form the dust in the first place.

While oxygen was detectable, ALMA didn't see signs of carbon, which could indicate that whatever carbon gas was present may have lost its electrons, becoming ionized. “Something unusual may be happening in this galaxy,” said Inoue. “I suspect that almost all the gas is highly ionized.”

If that's the case, it would be relatively easy for ionizing light to escape from the galaxy. Indeed, the researchers estimate that roughly fifty percent of the ionizing photons can escape. That’s enough to ionize the rest of the Universe, the team concludes: “This ionizing photon emission efficiency is strong enough to reach (or even exceed) the cosmic ionizing photon emissivity at z ~ 7 estimated from various observational constraints on reionization,” they write in their paper.

Previous studies have looked for clues to these processes in more nearby (and thus more recent) galaxies, but it has been difficult to get good observations from such distant galaxies. This study proves that ALMA can be used to peer into that early epoch of the Universe's history, which means a door has been opened for plenty of new research.

Some of that new research will be done by the same team, which has already started its next series of observations with ALMA. “Higher resolution observations will allow us to see the distribution and motion of ionized oxygen in the galaxy and provide precious information to understand the properties of the galaxy,” said Yoichi Tamura of the University of Tokyo and another of the paper's authors.

Science, 2015. DOI: doi:10.1126/science.aaf0714 (About DOIs)