Reionization Simulations: Overview

reionization snapshot

Snapshot of reionization from a large-scale simulation of Cosmic Reionization (f2000; one-cell thick), and a movie. Black dots indicate the ionizing sources in 5 Mpc thick slice around the one-cell slice shown.


The progress and geometry of reionization vs. time as seen at redshifted 21-cm emission line of hydrogen, and a corresponding movie.

The first stars formed in the universe about 100 million years after the Big Bang. Before their formation, and ever since the cosmic recombination some 100,000 years after the Big Bang, the universe was cold, dark and neutral, which epoch is often referred to as “Dark Ages”. The tiny fluctuations in the matter distribution left by inflation started growing, which eventually lead to the formation of the first nonlinear structures, the halos, filaments and sheets of the Cosmic Web. The gas iniside halos was at this point sufficiently dense to cool further, through atomic line and/or molecular cooling, resulting in the formation of the first stars.

The ultraviolet radiation from these first stars propagated ionization fronts into the surrounding gas, gradually ionizing and heating it. Eventually, by the time the universe was about 1 billion years old the whole universe became highly ionized, as evidenced by the spectra of the earliest observed QSO’s and galaxies. This process is referred to as Cosmic Reionization. As the above images show, this process was quite inhomogeneous, with some regions (generally the denser ones) becoming ionized much earlier than others. The ionizing sources responsible for this process were generally quite small (by the standards of the present-day Universe) halos, with total mass of the order of billion Solar masses. However, they were also quite numerous, since the cosmological structures in the Cold Dark Matter model form hierarchically, with the smallest halos forming first and then merging into larger ones. These early halos were also very highly-clustered, thus their combined effect was to produce the very large ionized regions shown above.

The Cosmic Reionization epoch is quite difficult to observe, due to its remoteness from us and the weakness of the signals coming from it. Recently a number of novel observational approaches have been proposed, some of the most prominent being: 1) the redshifted 21-cm line of neutral hydrogen, 2) the small-scale temperature anisotropies of the Cosmic Microwave Background created by reionization, and 3) Lyman-α emitter surveys at high redshift.

Our group has performed the first large-scale radiative transfer simulations of this process, using the state-of-the-art codes

  • PMFAST for N-body simulations, developed by Hugh Merz, Ue-Li Pen and Hy Trac
  • C2-Ray for the radiative transfer, developed by Garrelt Mellema, Ilian Iliev, Marcelo Alvarez and Paul Shapiro.

The output of the simulations are processed using special Fortran and IDL routines and analysis programmes and allow for the first realistic predictions of the reionization observables at the relevant scales. Details about these simulations can be found in the following papers:

Simulating cosmic reionization at large scales - I. The geometry of reionization by Ilian Iliev, Garrelt Mellema, Ue-Li Pen, Hugh Merz, Paul Shapiro and Marcelo Alvarez : Discusses our simulation methodology and basic results for the character of the global reionization process.

Simulating Cosmic Reionization at Large Scales II: the 21-cm Emission Features and Statistical Signals by Garrelt Mellema, Ilian Iliev, Ue-Li Pen and Paul Shapiro: Discusses the various 21-cm line signals from reionization.

Self-Regulated Reionization by Ilian Iliev, Garrelt Mellema, Paul Shapiro and Ue-Li Pen: discusses the self-regulating character of reionization due to Jeans-mass filtering of low-mass galaxies.

The Kinetic Sunyaev-Zel'dovich Effect from Radiative Transfer Simulations of Patchy Reionization by Ilian Iliev, Ue-Li Pen, J. Richard Bond, Garrelt Mellema and Paul Shapiro: discusses the the small-scale temperature anisotropies of the Cosmic Microwave Background created by reionization through kinetic Sunyaev-Zeldovich effect which is due to Compton scattering of the Cosmic Microwave Background photons on moving free electrons.

Reionization observables after WMAP 3-year results by Ilian Iliev, Garrelt Mellema, Ue-Li Pen, J. Richard Bond, and Paul Shapiro: discusses the dependence of redshifted 21-cm emission and kSZ from pachy reionization on the basic cosmological parameters and various observational issues.

The Signature of Patchy Reionization in the Polarization Anisotropy of the CMB by Olivier Dore, Gil Holder, Marcelo Alvarez, Ilian Iliev, Garrelt Mellema, Ue-Li Pen, and Paul Shapiro: derives the CMB polarization anisotropy due to reionization.


Click here for details and visualizations from our simulations.

reionization_sims.txt · Last modified: 2007/03/30 19:14 by iliev
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