Out of the Dark Ages

Light from the most distant sources known, emitted when the Universe was only a billion years old, hints at a complex history of star and galaxy formation, and at their effect on the primordial gas around them.

The appearance of the first sources of light in the Universe ended the so-called cosmic Dark Ages, which had lasted a few hundred million years. The ultraviolet light from these sources also changed the physical state of the gas (hydrogen and helium) that fills the Universe, from a neutral state to a nearly fully ionized one. This was the era of reionization. Quasars — the brightest and most distant objects known (Fig. 1) — offer a window on the reionization era, because neutral hydrogen gas absorbs their ultraviolet light. On page 815 of this issue, Wyithe and Loeb1 provide a new interpretation of the hydrogen-absorption spectra of the most distant quasars known. They conclude that a significant fraction of the cosmic hydrogen was still neutral about a billion years after the Big Bang. Coupled with other observations, their analysis adds to a growing body of evidence that the early history of galaxy formation was more complex than was supposed even a couple of years ago.

Figure 1: A long time ago in a galaxy far, far away.


This image from the W. M. Keck 10-m telescope shows the quasar SDSS J1148+5251 as it was about 12.8 billion years ago: at a redshift of 6.41, it is currently the most distant quasar known. The spectra of ultraviolet light from this and other quasars at comparable redshifts have been used by Wyithe and Loeb1 to constrain the reionization history of the Universe.

When the Universe was about 380,000 years old, it underwent a phase transition, changing from an incandescent plasma containing the heat of the Big Bang, to a space filled with dark matter, energy and neutral gas. The glow of the primordial plasma is what we now observe as the cosmic microwave background (CMB). The Universe then entered the Dark Ages, with embryonic structures growing from the seeds of dark-matter fluctuations (fluctuations that are observed today as ripples in the CMB). After a few hundred million years, these condensations became dense enough for the first stars to form, leading to the appearance of the first galaxies and the growth of the massive black holes that are believed to power quasars.

As these first objects lit up, they also modified the gas between them, ionizing the hydrogen and making it transparent to ultraviolet light. Effectively, the Universe underwent another phase transition, from a neutral to an ionized state. Each of the primordial sources of light — most of them powered by young, massive stars, but some powered by the accretion of matter into growing black holes (the early quasars) — excavated a bubble of ionized gas, called a Strömgren sphere, in the otherwise neutral surrounding medium. When these bubbles began to overlap, reionization was complete. The reionization era is thus a cosmological milestone, marking the appearance of the first stars, galaxies and quasars.

The light reaching us now from distant quasars was emitted just before reionization was complete. Any neutral hydrogen remaining in the Universe at that point would have been a very effective absorber of this radiation at a particular set of wavelengths (extending up to 91.2 nanometres) known as the Lyman series. Even minuscule quantities of neutral hydrogen — as little as 10−4 or 10−5 by mass fraction in intergalactic clouds — can leave a tell-tale absorption signature in the spectra of quasars; this is the ‘Lyman α forest’ of absorption lines in quasar spectra. As the fraction of the neutral gas increases, the forest thickens; as the fraction approaches 100%, essentially all of the observed flux at wavelengths below the so-called Lyman α line (at a wavelength of 121.6 nanometres, in the restframe) is absorbed.

The existence of such an extended trough in the spectra of quasars was predicted in 1965 by Gunn and Peterson2, and was indeed detected3,4 in the spectra of quasars at redshifts greater than 6. (Redshift is a measure of how much the Universe has expanded since the light was emitted; a redshift of 6 corresponds to a time roughly one billion years after the Big Bang in the favoured cosmological models.) Analysis5 of the spectra of the handful of quasars now known at such redshifts indicates that there is an abrupt change in the physical state of the intergalactic gas at this epoch. However, the actual fraction of the neutral gas is hard to constrain, because it involves measurements of fluxes that are very close to zero. Even with the best data available, all we can say is that the neutral fraction is at least 10−3, which is not a very strong limit.

But a surprise was provided by the measurement of the polarized fluctuations of the CMB by the Wilkinson Microwave Anisotropy Probe (WMAP)6. These data indicated that the Universe was reionized at a redshift of 10 to 20 — that is, some 200 million to 500 million years after the Big Bang. How can this be reconciled with the quasar data, which suggest that the last stages of reionization occurred after one billion years?

Several authors7,8 have proposed that the Universe was in fact reionized twice. The first time was by a generation of massive luminous stars, and this gives rise to the WMAP result. But the radiation from these stars and their eventual explosions as supernovae could have disrupted further star formation in their vicinity, leading to a hiatus during which ionized gas in the Universe might have recombined to become neutral again. A later generation of stars in young galaxies and quasars might have completed the reionization, at the end of the cosmic Dark Ages, when the Universe was about a billion years old.

Wyithe and Loeb1 have put a clever new twist on the interpretation of the quasar spectra in which the Gunn–Peterson absorption effect is seen. They have used the extent of the ionized regions around the quasars themselves to derive a fresh constraint on the fraction of the neutral hydrogen at this time. A quasar will excavate its own bubble of ionized gas in the neutral gas around it. The extent of this bubble would depend on several factors, including the history of the quasar's own luminosity, and the neutral-gas fraction in the surrounding medium (more neutral gas would mean a smaller bubble radius). By making reasonable assumptions and modelling the possible growth history of these quasars, Wyithe and Loeb conclude that, as the observed ionized bubbles around quasars seem small, the bubbles must have been embedded in a medium with a high fraction of neutral gas — as much as tens of per cent when the quasars turned on.

Coupled with the WMAP result, this implies that the Universe was indeed reionized at least twice. At any rate, the history of early galaxy formation and its effects on the primordial intergalactic medium seem complex. In our attempts to understand the reionization of the Universe, and the end of the cosmic Dark Ages, there are probably many more surprises to come.


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Djorgovski, S. Out of the Dark Ages. Nature 427, 790–791 (2004).

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