THE EPOCH OF REIONIZATION
After the cosmic microwave background formed 400,000 years after the Big Bang, all the Universe's hydrogen gas was in atomic form: an electron orbiting each proton. But this did not last: massive stars produce enough energy to tear apart, or ionize, hydrogen, separating protons and electrons. When galaxies grew in the first billion years, light from their hot stars spread into the surrounding intergalactic gas, producing bubbles of ionized gas around them. As galaxies continued to grow, so did the ionized bubbles – reionizing all hydrogen. This is the landmark event of the first generations of galaxies, marking the transition from the Cosmic Dawn to the later evolution of "normal" galaxies.
Star cluster NGC 3603, a modern example of a young cluster surrounded by gas and dust, helping us understand massive star formation in the early universe.
Image Credit: NASA, ESA, R. O'Connell, F. Paresce, E. Young, WFC3 Science Oversight Committee, and Hubble Heritage Team
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How is hydrogen ionized?

Hydrogen is the simplest of all atoms: normal hydrogen consists of a single proton, with a positive electric charge, and a single electron, with a negative charge. The mutual electrical attraction of these particles holds the atom together, but if light with enough energy hits the atom, it can knock the electron free. This is called ionization. At the beginning of the Cosmic Dawn – during the Universe’s Dark Ages – nearly all of its hydrogen was electrically neutral, with a proton and an electron bound together. But stars – at least those much more massive than our own Sun – produce a great deal of ionizing light, and can tear these atoms apart.

Image: IC 342, and its core which is a specific type of central region known as an HII nucleus - a name that indicates the presence of ionized hydrogen - that is likely to be creating many hot new stars.
Credit: ESA/Hubble, NASA

How did galaxies reionize the Universe?

All galaxies produce massive stars, which in turn produce ionizing light. But most stars are formed deep inside their galaxies, and today the hydrogen gas of those galaxies absorbs nearly all the ionizing light, preventing it from escaping. However, during the Cosmic Dawn, astronomers believe that the small galaxies were violent places, full of swirling gas that occasionally opened up escape paths for that ionizing light. Once it escaped, that light would be absorbed by hydrogen gas surrounding the galaxy – making a bubble of ionized gas around each galaxy. But these galaxies were common enough that those bubbles quickly overlapped. As more and more galaxies formed, and as each galaxy grew larger, their bubbles grew and merged, until they filled the Universe. This is reionization: the last time in the history of the Universe when hydrogen throughout the Universe changed its state. Studying the process will help us learn both about the hydrogen gas pervading the Universe and about the galaxies driving reionization – even those we cannot see individually.

Image: This infrared image from NASA's Spitzer Space Telescope shows in the Milky Way in the constellation Aquila, a cloud of gas and dust full of bubbles, which are inflated by wind and radiation from massive young stars.
Credit: JPL, NASA/JPL-Caltech

When did reionization occur?

Astronomers have worked hard over the past twenty years to understand reionization, and they have learned a great deal about when it happened (about a billion years after the Big Bang). The hydrogen gas itself produces only the weakest of light (in the spin-flip background, which has not yet been observed in this period), these techniques are all based on finding a distant source of light and observing how the intervening gas affects that source's light. For details on some such methods, see below. But we cannot yet study the how of reionization: no observations exist yet that can probe the growth of ionized bubbles in the Universe.

Image: 'Snapshots' of reionization according to a computer simulation.
Credit: McQuinn et al. 2007, The Astrophysical Journal, vol. 377, pp. 1043-1063.

How does reionization affect the Universe?

Reionization is fascinating on its own terms, because it teaches us both about galaxies during the Cosmic Dawn and the remaining matter at that time - when only about 5% of the Universe's hydrogen was inside a galaxy! But it also has important implications for later generations of galaxies. In addition to ionizing the gas, reionization also heats it - likely by a factor of several thousand. The increased gas temperature makes it harder for other galaxies, especially small ones, to attract this gas. Thus reionization shifts galaxy growth toward larger objects, halting some of the smallest from continuing to form stars. This effect may even explain the properties of some of the Milky Way galaxy's smaller neighbors.

Image: The Large Magellanic Cloud, a satellite galaxy of the Milky Way, floats in space, in a long and slow dance around our galaxy.
Credit: NASA Goddard

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A Closer Look: The Physics of Reionization

How can we learn about galaxies by studying reionization?

Ionized Bubbles Grow: The sizes of the ionized bubbles that grow during reionization depend on the galaxies producing them, in both obvious and subtle ways. By studying the size distribution of these bubbles, we can therefore learn about the galaxies themselves.
This image shows some 'snapshots' of reionization according to a computer simulation. Each panel shows the mixture of ionized (white) and neutral (black) gas. Each row takes a different phase of reionization (early, middle, late, top to bottom), and each column uses a different set of galaxies to drive reionization, with the size of the galaxies increasing from left to right.
If we follow one of these computer simulations through the stages, we see the ionized bubbles growing as they complete reionization. The first thing we learn by measuring the bubble sizes is the timing of reionization.

Ionized bubbles also depend on the galaxies that produce them: However, if we compare all the snapshots in an individual row – all taken at the same point during reionization – we also see that the bubbles grow bigger as the galaxies grow bigger! This is not for what might seem the most obvious reason – after all, bigger galaxies have more stars so make more ionizing light. Ionized bubbles have hundreds or thousands of sources – even the largest galaxy can only make a very small bubble. Instead, it is because the largest galaxies are most likely to have neighbors, which can combine forces to create even larger bubbles. Thus the second thing we learn about reionization is how large the galaxies driving it are.
This is particularly useful because, in most predictions of the Cosmic Dawn, the vast majority of the starlight is produced by extremely small galaxies that we cannot otherwise easily detect. But by studying the reionization process, we can infer their properties even without seeing them!

Figure Credit: McQuinn et al. 2007, The Astrophysical Journal, vol. 377, pp. 1043-1063.

What are some methods of determining when reionization occurred?

CMB Scattering: The cosmic microwave background (CMB) was produced 400,000 years after the Big Bang, so on its way to our telescopes the microwave light must pass through the neutral gas (before reionization) and the ionized gas (afterward). The neutral hydrogen has no effect on most of the cosmic microwave background, but once the electrons are liberated from their atoms, they scatter the microwave light. Each time a microwave collides with an electron, the path of the microwave is bent slightly, blurring out the cosmic microwave background just a bit. But this scattering also imprints a direction to the oscillation of the light, called polarization (white lines). Satellites like the Wilkinson Microwave Anisotropy Probe and Planck have measured the amount of polarization. The latter estimates that reionization reached its midpoint roughly 700 million years after the Big Bang.

Galaxy Light Absorption: Hydrogen absorption can also be seen via galaxies - although normal galaxies are far fainter than quasars, they are also far more common. In the future, it may be possible to map out the ionized bubbles by looking at how intervening hydrogen affects their light. But, for now, this is challenging because of how little we understand the galaxies.

Ultraviolet Absorption: Although neutral hydrogen doesn't have a significant effect on microwave photons, it can strongly absorb ultraviolet light. Astronomers can search for the signatures of this absorption on light from sources from before the end of reionization. This is easiest to do with extremely bright sources, such as quasars (whose exceptional luminosity is generated by gas falling onto supermassive black holes). Recent analyses of two of these objects also suggest that reionization was ongoing about 700 million years after the Big Bang. Meanwhile, measurements of many other quasars that existed about a billion years after the Big Bang suggest that reionization is ending at roughly that time.

Image: This shows the polarization of the CMB observed by the WMAP satellite, which measures when reionization occurred.
Credit: NASA / WMAP Science Team

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