For a few instants after the Big Bang, the Universe was hot and dense enough to fuse hydrogen into helium and lithium. But this period of time was transitory. Heavier elements, including oxygen, carbon, and the like, didn't have time to form; they were the product of stars that only appeared hundreds of millions of years later. Modern galaxies still contain ancient stars with a composition corresponding to early times. However, there is a significant gap in our knowledge: no direct detailed chemical data from the first billion years of the Universe has been available.

Now a study of a quasar from 772 million years after the Big Bang has helped fill in that gap. Robert A. Simcoe and colleagues measured the absorption of light by gas immediately around the quasar, and determined it to be nearly devoid of elements heavier than helium. Their results leave two possibilities. If the hydrogen gas was part of an early galaxy, then it could have been part of the environment in which early (as yet unobserved) stars could form. Alternatively, if the gas was part of intergalactic space, it indicates that the first stars formed somewhat later than many models predicted.

The process of building chemical elements via nuclear fusion is known as nucleosynthesis. During the first few minutes after the Big Bang, conditions were right to produce most of the hydrogen, helium, and lithium in the cosmos, a process called Big Bang nucleosynthesis (BBN). Heavier elements, known (perversely) by astronomers as "metals", were later produced in stars via stellar nucleosynthesis, and distributed into interstellar space as those stars died.

Modern galaxies like the Milky Way contain populations of stars that we can divide based on their metal content. The Sun is a Population I star, with a relatively high metal abundance; older, Population II stars near the galactic center are metal-poor. However, the earliest, metal-free stars—known as Population III—are still hypothetical. According to widely accepted models, Population III stars were massive and therefore short-lived, going supernova and spreading the first metals into interstellar space.

When did these first stars form, and did they actually correspond to our models? These questions are still unanswered. The crucial period of time when the first stars must have formed is still marked by a paucity of data.

The researchers in the present study used the Baade telescope, one of the twin Magellan telescopes at the Las Campanas Observatory in Chile. They analyzed the infrared spectrum of the quasar ULAS J1120+0641, which emitted its light 772 million years after the Big Bang. (Quasars are bright beams of light emitted by supermassive black holes; they are some of the only objects that can be seen at such vast distances.)

Comparing the predicted spectrum of an ideal quasar to that of ULAS J1120+0641, the astronomers found an anomalously large amount of hydrogen absorption from neutral hydrogen atoms in the quasar's immediate environment, and no significant indication of metals.

The unusual aspects of the spectrum indicate either a high density of neutral hydrogen very close to the quasar, or a more diffuse cloud around it. These options carry very different implications. If the hydrogen is close to the supermassive black hole, then it likely was part of a protogalaxy: a collapsing cloud of gas and dark matter that would eventually form a galaxy. In that case, the total environment would be conducive to the formation of Population III stars (though the Magellan data would not be sufficient to observe them).

On the other hand, if the neutral hydrogen formed a diffuse cloud, then it would suggest the first stars didn't form until a later date. If stars were already present at the time the quasar emitted this light, their output would ionize the hydrogen, changing the absorption spectrum.

Of course, one quasar doesn't provide a definitive case study of the entire environment 772 million years after the Big Bang. The earliest stars didn't all form at once, so it's unwise to extrapolate from this one quasar to the entire early Universe. Nevertheless, this observation provides an interesting preliminary case study, bringing us closer to a picture of the environment in which the first stars formed.

Nature, 2012. DOI: 10.1038/nature11612 (About DOIs).