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Previously we identified a new class of early galaxy that we estimate contributes up to 30% of the ionizing photons responsible for reionization. These are low-mass halos in the range = 106.5–108 that have been chemically enriched by supernova ejecta from prior Population III star formation. Despite their low star formation rates, these metal cooling halos (MCs) are significant sources of ionizing radiation, especially at the onset of reionization, owing to their high number density and ionizing escape fractions. Here we present a fully coupled radiation hydrodynamic simulation of reionization that includes these MCs, as well the more massive hydrogen atomic line cooling halos. Our method is novel: we perform halo finding in line with the radiation hydrodynamical simulation and assign escaping ionizing fluxes to halos using a probability distribution function (pdf) measured from the galaxy-resolving Renaissance Simulations. The pdf captures the mass dependence of the ionizing escape fraction, as well as the probability that a halo is actively forming stars. With MCs, reionization starts earlier than if only halos of 108 and above are included; however, the redshift when reionization completes is only marginally affected, as this is driven by more massive galaxies. Because star formation is intermittent in MCs, the earliest phase of reionization exhibits a stochastic nature, with small H ii regions forming and recombining. Only later, once halos of mass ∼109 and above begin to dominate the ionizing emissivity, does reionization proceed smoothly in the usual manner deduced from previous studies. This occurs at z ≈ 10 in our simulation. [ABSTRACT FROM AUTHOR]

Cosmic reionization is thought to be primarily fueled by the first generations of galaxies. We examine their stellar and gaseous properties, focusing on the star formation rates and the escape of ionizing photons, as a function of halo mass, redshift, and environment using the full suite of the Renaissance Simulations with an eye to provide better inputs to global reionization simulations. This suite probes overdense, average, and underdense regions of the universe of several hundred comoving Mpc3, each yielding a sample of over 3000 halos in the mass range of 107–109.5 at their final redshifts of 15, 12.5, and 8, respectively. In the process, we simulate the effects of radiative and supernova feedback from 5000 to 10,000 Population III stars in each simulation. We find that halos as small as 107 are able to host bursty star formation due to metal-line cooling from earlier enrichment by massive Population III stars. Using our large sample, we find that the galaxy-halo occupation fraction drops from unity at virial masses above 108.5 to ∼50% at 108 and ∼10% at 107 , quite independent of redshift and region. Their average ionizing escape fraction is ∼5% in the mass range of 108–109 and increases with decreasing halo mass below this range, reaching 40%–60% at 107 . Interestingly, we find that the escape fraction varies between 10%–20% in halos with virial masses of ∼3 × 109 . Taken together, our results confirm the importance of the smallest galaxies as sources of ionizing radiation contributing to the reionization of the universe. [ABSTRACT FROM AUTHOR]

We present results on the formation of Population III (Pop III) stars at redshift 7.6 from the Renaissance Simulations, a suite of extremely high-resolution and physics-rich radiation transport hydrodynamics cosmological adaptive-mesh refinement simulations of high-redshift galaxy formation performed on the Blue Waters supercomputer. In a survey volume of about 220 comoving Mpc3, we found 14 Pop III galaxies with recent star formation. The surprisingly late formation of Pop III stars is possible due to two factors: (i) the metal enrichment process is local and slow, leaving plenty of pristine gas to exist in the vast volume; and (ii) strong Lyman–Werner radiation from vigorous metal-enriched star formation in early galaxies suppresses Pop III formation in (“not so”) small primordial halos with mass less than ∼3 × 107M⊙. We quantify the properties of these Pop III galaxies and their Pop III star formation environments. We look for analogs to the recently discovered luminous Ly α emitter CR7, which has been interpreted as a Pop III star cluster within or near a metal-enriched star-forming galaxy. We find and discuss a system similar to this in some respects, however, the Pop III star cluster is far less massive and luminous than CR7 is inferred to be. [ABSTRACT FROM AUTHOR]

We present our prediction for the local 21 cm differential brightness temperature (δTb) from a set of strongly clustered sources of Population III (Pop III) and II (Pop II) objects in the early universe, by a numerical simulation of their formation and radiative feedback. These objects are located inside a highly biased environment, which is a rare, high-density peak (“Rarepeak”) extending to ∼7 comoving Mpc. We study the impact of ultraviolet and X-ray photons on the intergalactic medium (IGM) and the resulting δTb, when Pop III stars are assumed to emit X-ray photons by forming X-ray binaries very efficiently. We parameterize the rest-frame spectral energy distribution of X-ray photons, which regulates X-ray photon-trapping, IGM-heating, secondary Lyα pumping and the resulting morphology of δTb. A combination of emission (δTb > 0) and absorption (δTb < 0) regions appears in varying amplitudes and angular scales. The boost of the signal by the high-density environment (δ ∼ 0.64) and on a relatively large scale combines to make Rarepeak a discernible, spatially extended (θ ∼ 10′) object for 21 cm observation at 13 ≲ z ≲ 17, which is found to be detectable as a single object by SKA with integration time of ∼1000 hr. Power spectrum analysis by some of the SKA precursors (Low Frequency Array, Murchison Widefield Array, Precision Array for Probing the Epoch of Reionization) of such rare peaks is found to be difficult due to the rarity of these peaks, and the contribution only by these rare peaks to the total power spectrum remains subdominant compared to that by all astrophysical sources. [ABSTRACT FROM AUTHOR]