The Group






   Research Interests:
The High-Redshift Universe

Reionization is the process by which galaxies and quasars ionized hydrogen in the intergalactic medium (IGM). It is a milestone event in the history of the Universe, because it marks the time at which structure formation affected every baryon in the Universe. Beyond that point, the IGM became transparent to ultraviolet photons, allowing galaxies to "see" each other all the better.

Observations now suggest that reionization occurred sometime around redshifts 6.5-12 (or about 500 million years-1 billion years after the Big Bang), although the uncertainties are still large (see the Figure at right, taken from our recent review, which shows ). The most powerful probes to date are high-redshift quasars found by the Sloan Digital Sky Survey and measurements of the cosmic microwave background (CMB) by the Wilkinson Microwave Anisotropy Probe. Unfortunately, neither can yet provide a decisive description of reionization: quasars are too rare, and the CMB provides only an integrated background measure.

This is unfortunate because reionization offers a wealth of information on the first sources of light in the Universe. In particular, the "geometry" of reionization - how large the HII regions surrounding galaxies and quasars grow, and how they relate to the underlying density field, can teach us about star formation in the first galaxies and how those galaxies form from the IGM. Together with Matias Zaldarriaga and Lars Hernquist (as well as their students), I have developed a model predicting the sizes and locations of these ionized bubbles. The figure below shows three views of a simulation of reionization (courtesy O. Zahn) illustrating how the bubbles grow (the orange colorscale shows the density of neutral gas; black regions are ionized and blue dots show the locations of galaxies). The movie below shows the Universe as it is being ionized; the colorscale actually shows the brightnes in the 21 cm transition.

Our group focuses on understanding the principles behind the bubble geometry as well as what observations can teach us about it (and thus about the first galaxies and the IGM). Some of these methods are:

  • 21 cm Observations of the High-Redshift IGM: Probably the most exciting possibility is the 21 cm transition - so exciting that it deserves its own page!

  • Quasar and GRB Spectra: The so-called Lyman-alpha forest is an unparalleled probe of intergalactic hydrogen at moderate redshifts. Unfortunately, by z~6 neutral gas has become common enough that the absorption is completely saturated - even though less than 1% of the hydrogen is actually neutral! Thus this technique can only probe the late stages of reionization. However, it offers a much more detailed view of small scale structure than any other potential probe. It is therefore ideally suited to studying the end phases of reionization, when the fine structure of the cosmic web begins to control the evolution of the ionizing background. Moreover, the detail of the spectra allows us to study unique features of the ionizing background, such as the "proximity zone" around each quasar and the damping wing of absorption, as well as heavy elements like oxygen, which can also probe the ionizing background.

  • Lyman-alpha Galaxy Surveys: A complementary probe takes advantage of recent advances in high-redshift galaxy surveys that use their bright Lyman-alpha lines, which form because most ionizing photons are absorbed inside the galaxy and recycled into Lyman-alpha photons. If these photons travel through neutral gas, they will be absorbed and scattered away from the line of sight - thus, we expect this technique to fail if we push far enough back in reionization. However, if the galaxy sits inside a large ionized bubble, the photons can redshift out of the Lyman-alpha resonance before encountering any neutral gas. Thus these surveys offer the opportunity to map the bubble structure (albeit indirectly) because large bubbles will contain many Lyman-alpha-bright galaxies while small bubbles will contain few. However, telescope technology - and in particular the field of view - must improve significantly before such dreams can be realized fully.

  • CMB Measurements: The CMB also contains a wealth of information about reionization. On large scales, scattering by free electrons along the line of sight generates (relatively) large polarization - it is this signal that WMAP has measured. It constrains the total column of ionized gas and hence the mean reionization redshift. However, the ionized bubbles also induce an inhomogeneous scattering component through the "kinetic Sunyaev-Zeldovich effect" that can be observed in arcminute-scale temperature fluctuations. The next generation of ground-based CMB telescopes hope to observe this signal.