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Cosmic Reionization

Ultra-deep imaging with the Hubble Space Telescope has now uncovered galaxies within the first billion years of cosmic history. These galaxies represent some of the first objects ever formed, yet in terms of early galaxy formation they are only the tip of the iceberg. Most of the galaxies at this early epoch are too faint to study directly, even with Hubble. Galaxies, moreover, represent only a small fraction of all the matter in the Universe. Most of the matter instead resides in a vast network of dark matter and baryons known as the intergalactic medium (IGM). One of the primary aims of extragalactic astronomy is therefore to understand how galaxies form out of this network, and how galaxies, in turn, affect their environments. Of particular interest is how some of the very first galaxies took shape, and how they changed the properties of the large-scale IGM.

One of the key features of the IGM in the present day Universe is that the gas is highly ionized. This would not always have been the case, however. After the Big Bang, the gas would have cooled and become neutral as the Universe expanded. Only after the first luminous sources appeared and began to emit ionizing photons would hydrogen in the IGM have been re-ionized. We believe that this process occurred within the first billion years, and represents one of the most dramatic impacts that early galaxies had on the properties of the baryons in deep space.
In addition to direct searches for distant galaxies, reionization can be probed by analysing the evolution of the IGM. Observationally, the IGM is most readily studied in absorption against distant quasars. The spectra of these objects show a dense series of absorption lines, the Lyman-alpha forest, which arise from intergalactic neutral hydrogen. Quasar spectra also show signatures of rarer elements such as oxygen, carbon, and iron, thought to be located in enriched regions in and around galaxies. Together with increasingly sophisticated computer simulations, quasar absorption lines have given rise to the picture of the IGM as a ”cosmic web” of filaments and voids, where gravitational collapse of large-scale structure creates the dense regions where galaxies are born.

Determining how and when ”reionization” occurred is one of the main goals of observa- tional cosmology, with implications for the formation of the first luminous sources, the evolution of the IGM, and the properties of galaxies that exist today.

Your essay should cover the following points:
1. Describe how observations of quasar absorption lines have given rise to a picture of the IGM as a ”cosmic web.” What are the key observations and how have these observations been combined with numerical simulations to improve our understanding of large-scale structure? 2. Describe, in general terms, a physical picture of how reionization occurs. What are the leading candidates for the sources responsible for hydrogen reionization? Discuss the arguments in favor of these sources, as well as any potential drawbacks or challenges in light of what we currently know about when reionization happened and the sources present at that time.
3. What have we learned about reionization from the cosmic microwave background as seen by WMAP and Planck?
4. How do we know that the IGM is ionized? What is the current lower limit on the redshift of hydrogen reionization, and how is this determined? After reionization, what keeps the IGM ionized? Where do the ionizing photons come from, and how (if at all) does this change over time?

Suggested References:

Rauch, M. 1998, The Lyman-alpha Forest in the Spectra of QSOs, ARAA, 36, 267
Bechtold, J. 2001, Quasar Absorption Lines,
Fan, X., et al. 2006, Observational Constraints on Cosmic Reionization, ARAA, 44, 415
Zaroubi, S., 2012, The Epoch of Reionization (book chapter), ISBN 978-3-642-32361-4, Springer-Verlag Berlin Heidelberg, 2013, p. 45
Oesch, P., et al. 2013, Probing the Dawn of Galaxies at z 9-12: New Constraints from HUDF12/XDF and CANDELS Data,

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