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Second Lightby Leslie J. Sage (l.sage@us.nature.com)A First Glimpse of theEpoch of Reionization –or NotIassume that all my readers know about the Big Bang, whichseems to be the birth event of the present Universe. After thefirst few minutes, the Universe settled down into somethingthat would be at least partially recognizable to us, dominated byprotons, electrons, helium nuclei (alpha particles), and photons. Asit expanded over the next few hundred thousand years, it cooled tothe point where the electrons could join with the protons to makehydrogen, and with the helium nuclei to make atomic helium. Thisevent is called “decoupling,” because the photons (light) becamedecoupled from matter, allowing the light to stream freely. We seethis event recorded in the fluctuations in the microwave background.But, readers are probably not generally aware of the next “big event”in the history of the Universe – the epoch of reionization, where thehydrogen between the forming galaxies – the intergalactic medium –gets reionized. A first look using the 21-cm line of atomic hydrogenis reported in the December 9 issue of Nature by Judd Bowman ofArizona State University and Alan Rogers of Haystack Observatory.They find that the epoch must have lasted longer than ~3 millionyears, assuming it took place at z~11.The physics of recombination are clear and relativelystraightforward, which is why the WMAP results have ushered inthe era of “precision cosmology.” The seeds of the present Universe –clusters of galaxies, and sheets/filaments of clusters – were implantedat that time, and there is a fairly remarkable agreement betweensimulations and observations.By contrast, the epoch of reionization is inherently messyastrophysics. There is no clear agreement on when it started, thoughit does appear to have ended somewhere around a redshift of 6. Atthat time in the Universe’s life, there are complicated, non-linearrelationships between distance, time, and redshift. Because theastronomical observable is redshift, we astronomers tend to fall backon that. Let me illustrate this with a few numbers. The WMAPdata are interpreted to indicate that reionization took place roughlyaround redshift z~11. Bowman and Rogers found that the epochmust have lasted a redshift interval of Δz > 0.06. At a redshift of 11,this corresponds to a time of just 3 million years. But Δz = 0.06 ata redshift of 6 corresponds to 12 million years. So, I hope you willexcuse the need to use the jargon of redshift.The messiness arises because of multiple reasons. We donot know if the first stars formed outside galaxy-like structures,or inside. If on the outside, before galaxies started forming, thenthe first massive stars can start ionizing the intergalactic mediumaround them. However, if those stars formed inside galaxies, thenthe photons needed to ionize the hydrogen and helium in theintergalactic medium have to get outside the galaxies first – andyoung galaxies will contain lots of gas to intercept the photonsbefore they even get to the edge. Recent estimates of the escape rateplace it around five percent. We also do not know when the firstblack holes formed, and whether they did so in gas-rich or gas-poorenvironments, which returns us to the pesky escape problem. Theblack holes themselves will not ionize the gas, but they typically aresurrounded by extremely hot disks of accreting gas that emit lotsof x-rays. Brant Robinson of Caltech and colleagues have arguedin a recent review article (see the November 4 issue of Nature) thatthere were enough photons from the early galaxies to reionize theintergalactic medium, but at this stage my own belief is that thesituation remains unclear.The question of what is ionizing the Universe also is linked towhen the process began, and when it ended. Simulations of whenthe first stars formed are quite sensitive to various assumptions, and,as a result, they offer little real guidance as a way to disentangle thevarious factors. There is also the fact that reionization must take aperiod of time as the photons take time to travel from the regionswhere galaxies and stars are abundant, to the places farthest from thegalaxies.Bowman and Rogers took a very straightforward approachof looking at the 21-cm signal of atomic hydrogen, averaged overthe whole observable sky from a radio-quiet location in WesternAustralia. If reionization occurred rapidly, they would see a distinct“step” in their spectrum, which goes from ~100 MHz (redshift z~13)to ~200 MHz (redshift z~6). They did not, and this places a lowerlimit on the length of the epoch of some millions of years.In a sense, this answers a question that no one was asking,because no cosmologist thought that the epoch would be over thatrapidly. But Bowman and Rogers have also stolen a march on the $10billion+ proposed Square Kilometer Array that some astronomershave been pushing for a decade (and which was recently sidelined fora while by the Decadal Survey in the US, though other countries –including possibly Canada – are pressing on with preliminary work).The main scientific justification for the SKA is to study the epoch ofreionization.Such a study would face daunting technical challenges, butBowman and Rogers have shown that they can be overcome, at leastto some extent. However, teasing the complicated astrophysics ofsources, times, distances, and extinctions out of 21-cm spectra seemto me to be the bigger challenge. There are three SKA “pathfinder”telescopes that will in the interim attempt to study reionization. Lofar(lofar.org) is already operational, and the Murchison WidefieldArray (mwatelescope.org) will be finished in 2012. Finally, an arraycalled PAPER (Precision Array to Probe Epoch of Reionization) hasbeen discussed, but not yet funded or built. I would be happy to beshown to be wrong in a few years, if the pathfinder telescopes areable to do the disentangling.Leslie J. Sage is Senior Editor, Physical Sciences, for Nature magazineand a Research Associate in the Astronomy Department at the Universityof Maryland. He grew up in Burlington, Ontario, where even the brightlights of Toronto did not dim his enthusiasm for astronomy. Currentlyhe studies molecular gas and star formation in galaxies, particularlyinteracting ones, but is not above looking at a humble planetary object.February / février 2011Promoting Astronomy In CanadaJRASC35