We introduce a powerful seminumeric modelling tool, 21cmfast, designed to efficiently simulate the cosmological 21‐cm signal. Our code generates 3D realizations of evolved density, ionization, peculiar velocity and spin temperature fields, which it then combines to compute the 21‐cm brightness temperature. Although the physical processes are treated with approximate methods, we compare our results to a state‐of‐the‐art large‐scale hydrodynamic simulation, and find good agreement on scales pertinent to the upcoming observations (≳1 Mpc). The power spectra from 21cmfast agree with those generated from the numerical simulation to within 10s of per cent, down to the Nyquist frequency. We show results from a 1‐Gpc simulation which tracks the cosmic 21‐cm signal down from z= 250, highlighting the various interesting epochs. Depending on the desired resolution, 21cmfast can compute a redshift realization on a single processor in just a few minutes. Our code is fast, efficient, customizable and publicly available, making it a useful tool for 21‐cm parameter studies.

Detailed theoretical studies of the high-redshift universe, and especially reionization, are generally forced to rely on time-consuming N-body codes and/or radiative transfer algorithms. We present a method to construct seminumerical ''simulations,'' which can efficiently generate realizations of halo distributions and ionization maps at high redshifts. Our procedure combines an excursion-set approach with first-order Lagrangian perturbation theory and operates directly on the linear density and velocity fields. As such, the achievable dynamic range with our algorithm surpasses the current practical limit of N-body codes by orders of magnitude. This is particularly significant in studies of reionization, where the dynamic range is the principal limiting factor, because ionized regions reach scales of tens of comoving Mpc. We test our halo-finding and ionization-mapping algorithms separately against N-body simulations with radiative transfer and obtain excellent agreement. We compute the size distributions of ionized and neutral regions in our maps. We find even larger ionized bubbles than do purely analytic models at the same volume-weighted mean hydrogen neutral fraction,x H i , especially early in reionization. We also generate maps and power spectra of 21 cm brightness temperature fluctuations, which for the first time include corrections due to gas bulk velocities. We find that velocities widen the tails of the temperature distributions and increase small-scale power, although these effects quickly diminish as reionization progresses. We also include some preliminary results from a simulation run with the largest dynamic range to date: a 250 Mpc box that resolves halos with masses M ! 2:2 ; 10 8 M . We show that accurately modeling the late stages of reionization,x H i P 0:5, requires such large scales. The speed and dynamic range provided by our seminumerical approach will be extremely useful in the modeling of early structure formation and reionization.

We present new upper limits on the volume-weighted neutral hydrogen fraction,x HI , at z ∼ 5-6 derived from spectroscopy of bright quasars. The fraction of the Lyα and Lyβ forests that is "dark" (with zero flux) provides the only model-independent upper limit onx HI , requiring no assumptions about the physical conditions in the intergalactic medium or the quasar's unabsorbed UV continuum. In this work we update our previous results using a larger sample (22 objects) of medium-depth (∼ few hours) spectra of high-redshift quasars obtained with the Magellan, MMT, and VLT. This significantly improves the upper bound onx HI derived from dark pixel analysis tox HI ≤ 0.06+0.05 (1σ) at z = 5.9 andx HI ≤ 0.04+0.05 (1σ) at z = 5.6. These results provide robust constraints for theoretical models of reionization, and provide the strongest available evidence that reionization has completed (or is very nearly complete) by z ≈ 6.

The earliest generation of stars and black holes must have established an early 'Lyman-Werner' background (LWB) at high redshift, prior to the epoch of reionization. Because of the long mean free path of photons with energies hν < 13.6 eV, the LWB was nearly uniform. However, some variation in the LWB is expected due to the discrete nature of the sources, and their highly clustered spatial distribution. In this paper, we compute the probability distribution function (PDF) of the LW flux that irradiates dark matter (DM) haloes collapsing at high redshift (z ≈ 10). Our model accounts for (i) the clustering of DM haloes, (ii) Poisson fluctuations in the number of corresponding star-forming galaxies and (iii) scatter in the LW luminosity produced by haloes of a given mass (calibrated using local observations). We find that >99 per cent of the DM haloes are illuminated by an LW flux within a factor of 2 of the global mean value. However, a small fraction, ∼10 −8 to 10 −6 , of DM haloes with virial temperatures T vir 10 4 K have a close luminous neighbour within 10 kpc, and are exposed to an LW flux exceeding the global mean by a factor of >20, or to J 21,LW > 10 3 (in units of 10 −21 erg s −1 Hz −1 sr −1 cm −2 ). This large LW flux can photodissociate H 2 molecules in the gas collapsing due to atomic cooling in these haloes, and prevent its further cooling and fragmentation. Such close halo pairs therefore provide possible sites in which primordial gas clouds collapse directly into massive black holes (M BH ≈ 10 4−6 M ), and subsequently grow into supermassive (M BH 10 9 M ) black holes by z ≈ 6.

We present a new flexible Bayesian framework for directly inferring the fraction of neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR, z ∼ 6 − 10) from detections and non-detections of Lyman Alpha (Lyα) emission from Lyman Break galaxies (LBGs). Our framework combines sophisticated reionization simulations with empirical models of the interstellar medium (ISM) radiative transfer effects on Lyα. We assert that the Lyα line profile emerging from the ISM has an important impact on the resulting transmission of photons through the IGM, and that these line profiles depend on galaxy properties. We model this effect by considering the peak velocity offset of Lyα lines from host galaxies' systemic redshifts, which are empirically correlated with UV luminosity and redshift (or halo mass at fixed redshift). We use our framework on the sample of LBGs presented in Pentericci et al. (2014) and infer a global neutral fraction at z ∼ 7 of x hi = 0.59 +0.11 −0.15 , consistent with other robust probes of the EoR and confirming reionization is on-going ∼ 700 Myr after the Big Bang. We show that using the full distribution of Lyα equivalent width detections and upper limits from LBGs places tighter constraints on the evolving IGM than the standard Lyα emitter fraction, and that larger samples are within reach of deep spectroscopic surveys of gravitationally lensed fields and JWST NIRSpec.

With their long mean free paths and efficient heating of the intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal and ionization history of the Universe. Here we run several semi-numeric simulations of the Dark Ages and the Epoch of Reionization (EoR), including both X-rays and UV radiation fields, attempting to provide an intuitive framework for interpreting upcoming observations. We explore the impact of X-rays on various signals: (i) Reionization history: including X-rays results in an earlier, slightly more extended EoR. However, efficient thermal feedback from X-ray heating could yield an extended epoch in which the Universe was ≈ 10% ionized. (ii) Reionization morphology: a sizable (∼10%) contribution of X-rays to reionization results in a more uniform morphology, though the impact is modest when compared at the same global neutral fraction,x HI . Specifically, X-rays produce a dearth of fully neutral regions and a suppression of smallscale (k ∼ > 0.1Mpc −1 ) ionization power by a factor of ∼ < 2. However, these changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust indicator of an X-ray contribution to the EoR. (iii) The kinetic Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays decrease the kSZ power at l = 3000 by ≈ 0.5µK 2 . Our extreme model in which X-rays entirely drive reionization is the only one which is marginally consistent with the recent upper limits on this signal from the South Pole Telescope, assuming no thermal Sunyaev-Zel'dovich (tSZ) -dusty galaxy cross-correlation. Since this extreme model is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy cross-correlation. (iv) The redshifted 21cm signal: the impact of X-rays on the 21cm power spectrum during the advanced stages of reionization (x HI ∼ < 0.7) is modest, except in extreme, X-ray dominated models. The largest impact of X-rays is to govern the timing and duration of IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization. This results in a 21cm power spectrum which is ∼10-100 times higher atx HI ∼ > 0.9 than obtained from naive estimates ignoring this overlap. On the other hand, if thermal feedback is efficient, the resulting extended epoch between X-ray heating and reionization could provide a clean probe of the matter power spectrum in emission, at redshifts more accessible than the Dark Ages.

The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization (z=6-12), and to explore earlier epochs of our Cosmic Dawn (z∼30). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14 m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA's scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe. This 21cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an Executive SummaryThe Square Kilometre Array (SKA) will have a low frequency component (AA-low/SKA-low 1 ) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe (see SKA Memo 125). It is during this phase that the first building blocks of the galaxies that we see around us today, including our own Milky Way, were formed. It is a crucial period for understanding the history of the Universe and one for which we have currently very little observational data.We divide the period into two different phases based on the physical processes which affect the Intergalactic Medium. The first period, which we call the Cosmic Dawn, saw the formation of the first stars and accreting black holes, which changed the quantum state of the still neutral Intergalactic Medium. The second period, known as the Epoch of Reionization, is the one during which large areas between the galaxies were photo-ionized by the radiation produced in galaxies and which ended when the Intergalactic Medium had become completely ionized.Observations of the redshifted 21-cm line with SKA will provide a new and unique window on the entire period of Cosmic Dawn and Reionization. The signal is sensitive to the emergence of the first stellar populations, radiation from growing massive black holes and the formation of larger groups of galaxies and bright quasars. At the same time it maps the distribution of most of the baryonic matter in the Universe. The study of the redshifted 21cm line will teach us fundamental new things about the earliest phases of structure formation and cosmology. It even has the potential to lead to the discovery of new physical phenomena. Here we present an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.The redshifted 21cm signal will be analyzed with different techniques, which each come with their own requirements for the SKA: (i) Tomography, (ii) power-spectra and higher-order statistics, (iii) hydrogen absorption, (iv) global/total-intensity signal. Whereas all precursors/pathfinders aim to study the signal statistically through its power spectrum, SKA will be able to image the neutral hydrogen distribution directly and its focus will therefore be more on tomograph...