Early reionization by decaying particles and cosmic microwave background radiation

We study the reionization scenario in which ionizing UV photons emitted from decaying particle, in addition to usual contributions from stars and quasars, ionize the universe. It is found that the scenario is consistent with both the first year data of the Wilkinson Microwave Anisotropy Probe and the fact that the universe is not fully ionized until z \sim 6 as observed by Sloan Digital Sky Survey. Likelihood analysis revealed that rather broad parameter space can be chosen. This scenario will be discriminated by future observations, especially by the EE polarization power spectrum of cosmic microwave background radiation.Comment: 5 pages, 5 figures, fig 2, table 1, and some typos are correcte

Observing the Reionization Epoch Through 21 Centimeter Radiation

We study the observability of the reionization epoch through the 21 cm hyperfine transition of neutral hydrogen. We use a high-resolution cosmological simulation (including hydrodynamics) together with a fast radiative transfer algorithm to compute the evolution of 21 cm emission from the intergalactic medium (IGM) in several different models of reionization. We show that the mean brightness temperature of the IGM drops from dT_b~25 mK to dT_b~0.01 mK during overlap (over a frequency interval of ~25 MHz), while the root mean square brightness temperature fluctuations on small scales drop abruptly from ~10 mK before overlap to ~0.1 mK at the end of overlap. We show that 21 cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes.Comment: 10 pages, 7 figures, submitted to MNRA

Cosmic Reionisation by Stellar Sources: Population II Stars

We study the reionisation of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high resolution hydrodynamical simulations. Ionising fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x 10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in previous studies of this kind. Our calculations reveal that the reionisation process is sensitive to the inclusion of dim sources with masses below ~10^9 h^-1 M_sol. We present the results of our reionisation simulation assuming a range of escape fractions for ionising photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z~6 derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting model has an escape fraction of ~20% and causes reionisation to occur by z~8, although the IGM remains fairly opaque until z~6. In order to simultaneously match the observations from the z=6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modeled here, we require an evolving escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure

The Epoch of Helium Reionization

We study the reionization of Helium II by quasars using a numerical approach that combines 3D radiative transfer calculations with cosmological hydrodynamical simulations. Sources producing the ionizing radiation are selected according to an empirical quasar luminosity function and are assigned luminosities according to their intrinsic masses. We present models in which these parameters are varied and examine characteristics of the resultant reionization process that distinguish the various cases. In addition, we extract artificial spectra from the simulations and quantify statistical properties of the spectral features in each model. We find that the most important factor affecting the evolution of He II reionization is the cumulative number of ionizing photons that are produced by the sources. Comparisons between He II opacities measured observationally and those obtained by our analysis reveal that the available ranges in plausible values for the parameters provide enough leeway to provide a satisfactory match. However, one property common to all our calculations is that the epoch of Helium II reionization must have occurred at a redshift between 3 < z < 4. If so, future observational programs will be able to directly trace the details of the ionization history of helium and probe the low density phase of the intergalactic medium during this phase of the evolution of the Universe.Comment: 39 pages, including 13 figures, submitted to MNRA

Observing the reionization epoch through 21-centimetre radiation

We study the observability of the reionization epoch through the 21-cm hyperfine transition of neutral hydrogen. We use a high-resolution cosmological simulation (including hydrodynamics) together with a fast radiative transfer algorithm to compute the evolution of 21-cm emission from the intergalactic medium (IGM) in several different models of reionization. We show that the mean brightness temperature of the IGM drops from δT_b∼ 25 mK to ∼10−² mK during overlap (over a frequency interval Δν ∼ 25 MHz), while the root mean square fluctuations on small scales drop abruptly from 〈δT²_b〉¹/² ∼ 10 mK to ∼ 10⁻¹ mK at the end of overlap. We show that 21-cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes

Cosmic Reionisation by Stellar Sources: Population III Stars

We combine fast radiative transfer calculations with high resolution hydrodynamical simulations to study an epoch of early hydrogen reionisation by primordial stellar sources at redshifts 15<z<30. With relatively conservative assumptions, population III star formation proceeds in a self-regulated manner both locally and globally and, for a conventional LCDM cosmology, can significantly reionise the intergalactic medium between 15<z<20 as long as a large fraction of ionising photons can escape from these earliest galaxies. We then combine these results with our earlier work focusing on the role of population II stars in galaxies with virial temperatures >10^4K at redshifts 5<z< 20. Hence, we construct a complete reionisation history of the Universe which matches the Thomson optical depths as measured by the WMAP satellite as well as the evolution of the Gunn Peterson optical depth as seen in the asborption spectra of the higest redshift quasars. We find that even with conservative estimates for the impact of negative feedback mechanisms, primordial stellar sources contribute significantly to early reionisation. Future observations of a Thomson optical depth of tau_e>~0.13 would bolster the claim for the existence of population III stars similar to the ones studied here.Comment: 40 pages, 9 figure