Cosmic Reionization

4 - 29 April 2016

Benedetta Ciardi, Martin Haehnelt, Daniel Stark, Saleem Zaroubi

Despite much recent theoretical and observational progress in our understanding of the formation of early cosmic structure, many fundamental questions remain open. The Epoch of Reionization (EoR) is an extremely fascinating period, as it hosts a number of important processes which decisively shape the structure of the Universe we see today.

Only recently have observations of very distant objects (quasars, galaxies, gamma ray bursts) and of the cosmic microwave background (CMB) radiation allowed quantitative studies of this important period in the evolution of our Universe. These suggest that, while the tail end of cosmic reionization can be positioned at z~5-6, the process must have begun at much earlier times and so must have had a complex history.

Although since the late 1950s it has been recognized that neutral hydrogen in the intergalactic medium may be directly detectable in emission or absorption against the CMB radiation at the frequency corresponding to the redshifted neutral hydrogen 21cm line providing a direct probe of the era of cosmological reionization, only now radio interferometers such as LOFAR, MWA and PAPER are delivering the first data.

At the moment though there is still a dearth of direct observational evidence and lots of questions remain unanswered: what was the nature of the first ionizing sources? Were the first stars much more massive than those we observe today, and when/why did the transition from the so-called top-heavy mode to a normal mode of star formation occur? What is the impact of these processes on cosmic reionization and the subsequent structure formation process? Were there other contributions to the ionizing photon production (e.g. mini-quasars or exotic particles)? These and many other questions regarding the end of the Dark Ages remain, at best, only partially answered and are still the subject of a lively debate. To better understand such fundamental processes much theoretical and observational investigation is still needed.

Within the coming year or two, much of the observational data that will be needed to answer some of the above still open questions will be available. The Planck CMB project would have delivered its data and products, including new constraints on the Thomson scattering optical depth of the CMB off free electrons released during the EoR. The long awaited 21cm data is also expected to give its first results, and provide a glimpse on the history of the reionization process. Projects designed to detect Lyman-alpha emitters from high redshift galaxies, such as the wide field survey carried out by the HyperSupremeCam on Subaru, will also be well on their way. Several projects planning to detect the Cosmic Infrared Background, such as Siber, will have given constraints on the contribution of the stars prevalent during the EoR.

Workshop format

The MIAPP workshop will provide the perfect environment for researchers involved in different aspects of EoR studies (both theoretical and observational) to meet and discuss recent advances. The program will offer the possibility of discussing a variety of observations (CMB, LAEs, QSOs, Lyalpha forest, 21cm line) which hasn't been available up to now, and of identifying a set of constraints which should drive further progress in the theoretical modelling of cosmic reionization. 

In the framework outlined above, we expect, among others, to:

  • Compare and discuss available measurements of 21cm line with the aim of identifying the best strategies for observations and data analysis.
  • Discuss implications of data on high-z LAEs from the new HyperSuprimeCam on board the Subaru telescope. Can they provide more stringent constraints when used in combination with other observations?
  • Discuss implications of data from e.g. UKIDSS and VISTA surveys, which should obtain absorption spectra from quasars at very high-z, i.e. /z /> 7-8. What do we learn from the presence/lack of them? Can they provide more stringent constraints when used in combination with other observations?
  • Discuss progress and challenges of cosmological radiative transfer simulations of the epoch of reionization.
  • Identify key theoretical open questions, which can be answered with the aid of the new available data.