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

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 First Stars

We review recent theoretical results on the formation of the first stars in the universe, and emphasize related open questions. In particular, we discuss the initial conditions for Population III star formation, as given by variants of the cold dark matter cosmology. Numerical simulations have investigated the collapse and the fragmentation of metal-free gas, showing that the first stars were predominantly very massive. The exact determination of the stellar masses, and the precise form of the primordial initial mass function, is still hampered by our limited understanding of the accretion physics and the protostellar feedback effects. We address the importance of heavy elements in bringing about the transition from an early star formation mode dominated by massive stars, to the familiar mode dominated by low mass stars, at later times. We show how complementary observations, both at high redshifts and in our local cosmic neighborhood, can be utilized to probe the first epoch of star formation.Comment: 38 pages, 10 figures, draft version for 2004 Annual Reviews of Astronomy and Astrophysics, high-resolution version available at http://cfa-www.harvard.edu/~vbromm

Radiative Transfer Effects on the Lya Forest

Strong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the HI and HeII Lya forest as well as accurate IGM metallicity measurements. UVB fluctuations could arise both from the inhomogeneous distribution of the ionizing sources and/or from radiative transfer (RT) through the filamentary IGM. In this study we investigate, via numerical simulations, the role of RT effects such as shadowing, self-shielding and filtering of the ionizing radiation, in giving raise to a fluctuating UVB. We focus on possible detectable signatures of these effects on quantities derived from Lya forest spectra, as photoionization rate fluctuations, eta parameter (the HeII to HI column density ratio) distributions and the IGM temperature at redshift about 3. We find that RT induces fluctuations up to 60% in the UVB, which are tightly correlated to the density field. The UVB mean intensity is progressively suppressed toward higher densities and photon energies above 4 Ryd, due to the high HeII opacity. Shielding of overdense regions (Delta > 5) from cosmic HeII ionizing radiation, produces a decreaseing trend of eta with overdensity. Furthermore we find that the mean eta value inferred from HI-HeII Lya forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings.Comment: 13 pages, 10 figures, Accepted for publication in MNRA

Experimental Constraints on Self-consistent Reionization Models

A self-consistent formalism to jointly study cosmic reionization and thermal history of the IGM is presented. The model implements most of the relevant physics governing these processes, such as the inhomogeneous IGM density distribution, three different sources of ionizing photons (PopIII stars, PopII stars and QSOs), and radiative feedback. By constraining the free parameters with available data on redshift evolution of Lyman-limit systems, Gunn-Peterson and electron scattering optical depths, Near InfraRed Background (NIRB), and cosmic star formation history, we select a fiducial model, whose main predictions are: (i) H was completely reionized at z \approx 15, while HeII must have been reionized by z \approx 12. At z \approx 7, HeIII suffered an almost complete recombination as a result of the extinction of PopIII stars, as required by the interpretation of the NIRB. (ii) A QSO-induced complete HeII reionization occurs at z=3.5; a similar double H reionization does not take place due to the large number of photons above 1 Ryd from PopII stars and QSOs, even after PopIII stars have disappeared. (iii) Following reionization, the temperature of the IGM corresponding to the mean gas density is boosted to 15000 K. Observations of T_0 are consistent with the fact that He is singly ionized at z > 3.5, while they are consistent with He being doubly ionized at z < 3.5. This might be interpreted as a signature of (second) HeII reionization. (iv) Only 0.3% of the stars produced by z=2 need to be PopIII stars in order to achieve the hydrogen reionization. Such model not only relieves the tension between the Gunn-Peterson optical depth and WMAP observations, but also accounts self-consistently for all known observational constraints (abridged).Comment: Revised version. Accepted for publication in MNRA

Cosmic Microwave Background Polarization and reionization: constraining models with a double reionization

Neutral hydrogen around high-z QSO and an optical depth tau ~ 0.17 can be reconciled if reionization is more complex than a single transition at z ~ 6-8. Tracing its details could shed a new light on the first sources of radiation. Here we discuss how far such details can be inspected through planned experiments on CMB large-scale anisotropy and polarization, by simulating an actual data analysis. By considering a set of double reionization histories of Cen (2003) type, a relevant class of models not yet considered by previous works, we confirm that large angle experiments rival high resolution ones in reconstructing the reionization history. We also confirm that reionization histories, studied with the prior of a single and sharp reionization, yield a biased tau, showing that this bias is generic. We further find a monotonic trend in the bias for the models that we consider, and propose an explanation of the trend, as well as the overall bias. We also show that in long-lived experiments such a trend can be used to discriminate between single and double reionization patterns.Comment: 8 pages, 11 figures. Substantial rewriting, replaced with accepted version. To be published in A&

Simulating Reionization in Numerical Cosmology

The incorporation of radiative transfer effects into cosmological hydrodynamical simulations is essential for understanding how the intergalactic medium (IGM) makes the transition from a neutral medium to one that is almost fully ionized. Here, we present an approximate numerical method designed to study in a statistical sense how a cosmological density field is ionized by a set of discrete point sources. A diffuse background radiation field is also computed self-consistently in our procedure. The method requires relatively few time steps and can be employed with simulations having high resolution. We describe the details of the algorithm and provide a description of how the method can be applied to the output from a pre-existing cosmological simulation to study the systematic reionization of a particular ionic species. As a first application, we compute the reionization of He II by quasars in the redshift range 3 to 6.Comment: 37 pages, 7 figures, Submitted to New

Reionization History from Coupled CMB/21cm Line Data

We study CMB secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code CRASH. The reionization history is consistent with the WMAP Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for l > 4000 and reaches a maximum amplitude of l(l+1)C_l/2\pi ~ 1.6 x 10^{-13} on arcmin scale, i.e. l as large as several thousands. We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21cm line emission fluctuations obtained from the same simulations. The two signals are highly anti-correlated on angular scales corresponding to the typical size of HII regions (including overlapping) at the 21cm map redshift. We show how the CMB/21cm cross-correlation can be used to: (a) study the nature of the reionization sources, (b) reconstruct the cosmic reionization history, (c) infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities.Comment: 7 pages, 5 figures, MNRAS in pres

Globular cluster formation within the Aquarius simulation

The Aquarius project is a very high-resolution simulation capable of resolving the full mass range of potential globular cluster (GC) formation sites. With a particle mass mp= 1.4 × 104 M¿, Aquarius yields more than 100 million particles within the virial radius of the central halo which has a mass of 1.8 × 1012 M¿, similar to that of the Milky Way. With this particle mass, dark matter concentrations (haloes) that give rise to GCs via our formation criteria contain a minimum of ~2000 particles. Here, we use this simulation to test a model of metal-poor GC formation based on collapse physics. In our model, GCs form when the virial temperatures of haloes first exceed 104 K as this is when electronic transitions allow the gas to cool efficiently. We calculate the ionizing flux from the stars in these first clusters and stop the formation of new clusters when all the baryonic gas of the Galaxy is ionized. This is achieved by adopting reasonable values for the star formation efficiencies and escape fraction of ionizing photons which result in similar numbers and masses of clusters to those found in the Milky Way. The model is successful in that it predicts ages (peak age ~13.3 Gyr) and a spatial distribution of metal-poor GCs which are consistent with the observed populations in the Milky Way. The model also predicts that less than 5 per cent of GCs within a radius of 100 kpc have a surviving dark matter halo, but the more distant clusters are all found in dark matter concentrations. We then test a scenario of metal-rich cluster formation by examining mergers that trigger star formation within central gas discs. This results in younger (~7¿13.3 Gyr), more centrally located clusters (40 metal-rich GCs within 18 kpc from the centre of the host) which are consistent with the Galactic metal-rich population. We test an alternate model in which metal-rich GCs form in dwarf galaxies that become stripped as they merge with the main halo. This process is inconsistent with observed metal-rich globulars in the Milky Way because it predicts spatial distributions that are far too extended