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Register a new userSimulating Cosmic Reionization
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Ilian T. Iliev
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The Cosmic Dark Ages and the Epoch of Reionization constitute a crucial missing link in our understanding of the evolution of the intergalactic medium and the formation and evolution of galaxies. Due to the complex nature of this global process it is best studied through large-scale numerical simulations. This presents considerable computational challenges. The dominant contributors of ionizing radiation were dwarf galaxies. These tiny galaxies must be resolved in very large cosmological volumes in order to derive their clustering properties and the corresponding observational signatures correctly, which makes this one of the most challenging problems of numerical cosmology. We have recently performed the largest and most detailed simulations of the formation of early cosmological large-scale structures and their radiative feedback leading to cosmic reionization. This was achieved by running extremely large (up to 29 billion-particle) N-body simulations of the formation of the Cosmic Web, with enough particles and sufficient force resolution to resolve all the galactic halos with total masses larger than 10^8 Solar masses in computational volumes of up to (163 Mpc)^3. These results were then post-processed by propagating the ionizing radiation from all sources by using fast and accurate ray-tracing radiative transfer method. Both of our codes are parallelized using a combination of MPI and OpenMP and to this date have been run efficiently on up to 2048 cores (N-body) and up to 10000 cores (radiative transfer) on the newly-deployed Sun Constellation Linux Cluster at the Texas Advanced Computing Center. In this paper we describe our codes, parallelization strategies, scaling and some preliminary scientific results. (abridged)
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We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100/h Mpc)^3, while at the same time capturing the dwarf galaxies which are primarily responsible for reionization. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, C^2-Ray. The resulting electron-scattering optical depth is in good agreement with the first-year WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exibit a large scatter about the mean and do not describe the global reionization history well. The minimum reliable volume size for such predictions is ~30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcminute and larger scales. We find two populations of HII regions according to their size, numerous, mid-sized (~10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. (abridged)
We present the largest-volume (425 Mpc/h=607 Mpc on a side) full radiative transfer simulation of cosmic reionization to date. We show that there is significant additional power in density fluctuations at very large scales. We systematically investigate the effects this additional power has on the progress, duration and features of reionization, as well as on selected reionization observables. We find that comoving simulation volume of ~100 Mpc/h per side is sufficient for deriving a convergent mean reionization history, but that the reionization patchiness is significantly underestimated. We use jackknife splitting to quantify the convergence of reionization properties with simulation volume for both mean-density and variable-density sub-regions. We find that sub-volumes of ~100 Mpc/h per side or larger yield convergent reionization histories, except for the earliest times, but smaller volumes of ~50 Mpc/h or less are not well converged at any redshift. Reionization history milestones show significant scatter between the sub-volumes, of Delta z=0.6-1 for ~50 Mpc/h volumes, decreasing to Delta z=0.3-0.5 for ~100 Mpc/h volumes, and $Delta z$~0.1 for ~200 Mpc/h volumes. If we only consider mean-density sub-regions the scatter decreases, but remains at Delta z~0.1-0.2 for the different size sub-volumes. Consequently, many potential reionization observables like 21-cm rms, 21-cm PDF skewness and kurtosis all show good convergence for volumes of ~200 Mpc/h, but retain considerable scatter for smaller volumes. In contrast, the three-dimensional 21-cm power spectra at large scales (k<0.25 h/Mpc) do not fully converge for any sub-volume size. These additional large-scale fluctuations significantly enhance the 21-cm fluctuations, which should improve the prospects of detection considerably, given the lower foregrounds and greater interferometer sensitivity at higher frequencies. (abridged)
We present detailed predictions for the redshifted 21cm signal from the epoch of reionization. These predictions are obtained from radiative transfer calculations on the results of large scale (100/h Mpc), high dynamic range, cosmological simulations. We consider several scenarios for the reionization history, of both early and extended reionization. From the simulations we construct and analyze a range of observational characteristics, from the global signal, via detailed images and spectra, to statistical representations of rms fluctuations, angular power spectra, and probability distribution functions to characterize the non-gaussianity of the 21cm signal. (abbreviated abstract)
We explore the effect of cosmic radiative feedback from the sources of reionization on the thermal evolution of the intergalactic medium. We find that different prescriptions for this feedback predict quite different thermal and reionization histories. In spite of this, current data can not discriminate among different reionization scenarios. We find that future observations both from 21-cm and CMB experiments can be used to break the degeneracy among model parameters provided that we will be able to remove the foreground signal at the percent (or better) level.
We examine the reionization history of present-day galaxies by explicitly tracing the building blocks of halos from the Cosmic Reionization On Computers project. We track dark matter particles that belong to $z=0$ halos to trace the neutral fractions at corresponding positions during rapid global reionization. The resulting particle reionization histories allow us to explore different definitions of a halos reionization redshift and to account for the neutral content of the interstellar medium. Consistent with previous work, we find a systematic trend of reionization redshift with mass - present day halos with higher masses have earlier reionization times. Finally, we quantify the spread of reionization times within each halo, which also has a mass dependence.
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