ALBERT

Description: The birth of the first luminous sources and the ensuing epoch of reionization are best studied via the redshifted 21-cm emission line, the signature of the first two imprinting the last. In this work, we present a fully Bayesian method, hibayes , for extracting the faint, global (sky-averaged) 21-cm signal from the much brighter foreground emission. We show that a simplified (but plausible) Gaussian model of the 21-cm emission from the Cosmic Dawn epoch (15 z 30), parametrized by an amplitude $A_{\rm H\,\small {I}}$ , a frequency peak $\nu _{\rm H\,\small {I}}$ and a width $\sigma _{\rm H\,\small {I}}$ , can be extracted even in the presence of a structured foreground frequency spectrum (parametrized as a seventh-order polynomial), provided sufficient signal-to-noise (400 h of observation with a single dipole). We apply our method to an early, 19-min-long observation from the Large aperture Experiment to detect the Dark Ages, constraining the 21-cm signal amplitude and width to be $-890 \lt A_{\rm H\,\small {I}} \lt 0$ mK and $\sigma _{\rm H\,\small {I}} \gt 6.5$ MHz (corresponding to z 〉 1.9 at redshift z ~= 20) respectively at the 95-per cent confidence level in the range 13.2 〈 z 〈 27.4 (100 〉 〉 50 MHz).

Description: In this paper, we present calculations of the UV luminosity function (LF) from the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations project, which combines N -body, semi-analytic and seminumerical modelling designed to study galaxy formation during the Epoch of Reionization. Using galaxy formation physics including supernova feedback, the model naturally reproduces the UV LFs for high-redshift star-forming galaxies from z ~ 5 through to z ~ 10. We investigate the luminosity–star formation rate (SFR) relation, finding that variable SFR histories of galaxies result in a scatter around the median relation of 0.1–0.3 dex depending on UV luminosity. We find close agreement between the model and observationally derived SFR functions. We use our calculated luminosities to investigate the LF below current detection limits, and the ionizing photon budget for reionization. We predict that the slope of the UV LF remains steep below current detection limits and becomes flat at M UV –14. We find that 48 (17) per cent of the total UV flux at z ~ 6 (10) has been detected above an observational limit of M UV ~ –17, and that galaxies fainter than M UV ~ –17 are the main source of ionizing photons for reionization. We investigate the luminosity–stellar mass relation, and find a correlation for galaxies with M UV 〈 –14 that has the form $M_{\ast } \propto 10^{-0.47M_\mathrm{UV}}$ , in good agreement with observations, but which flattens for fainter galaxies. We determine the luminosity–halo mass relation to be $M_\mathrm{vir} \propto 10^{-0.35M_\mathrm{UV}}$ , finding that galaxies with M UV = –20 reside in host dark matter haloes of 10 11.0±0.1 M at z ~ 6, and that this mass decreases towards high redshift.

Description: We introduce meraxes , a new, purpose-built semi-analytic galaxy formation model designed for studying galaxy growth during reionization. meraxes is the first model of its type to include a temporally and spatially coupled treatment of reionization and is built upon a custom (100 Mpc) 3 N -body simulation with high temporal and mass resolution, allowing us to resolve the galaxy and star formation physics relevant to early galaxy formation. Our fiducial model with supernova feedback reproduces the observed optical depth to electron scattering and evolution of the galaxy stellar mass function between z = 5 and 7, predicting that a broad range of halo masses contribute to reionization. Using a constant escape fraction and global recombination rate, our model is unable to simultaneously match the observed ionizing emissivity at z 6. However, the use of an evolving escape fraction of 0.05–0.1 at z ~ 6, increasing towards higher redshift, is able to satisfy these three constraints. We also demonstrate that photoionization suppression of low-mass galaxy formation during reionization has only a small effect on the ionization history of the intergalactic medium. This lack of ‘self-regulation’ arises due to the already efficient quenching of star formation by supernova feedback. It is only in models with gas supply-limited star formation that reionization feedback is effective at regulating galaxy growth. We similarly find that reionization has only a small effect on the stellar mass function, with no observationally detectable imprint at M * 〉 10 7.5 M . However, patchy reionization has significant effects on individual galaxy masses, with variations of factors of 2–3 at z = 5 that correlate with environment.

Description: We introduce the Evolution Of 21 cm Structure (EOS) project: providing periodic, public releases of the latest cosmological 21 cm simulations. 21 cm interferometry is set to revolutionize studies of the Cosmic Dawn (CD) and Epoch of Reionization (EoR). Progress will depend on sophisticated data analysis pipelines, initially tested on large-scale mock observations. Here we present the 2016 EOS release: 1024 3 , 1.6 Gpc, 21 cm simulations of the CD and EoR, calibrated to the Planck 2015 measurements. We include calibrated, sub-grid prescriptions for inhomogeneous recombinations and photoheating suppression of star formation in small-mass galaxies. Leaving the efficiency of supernovae feedback as a free parameter, we present two runs which bracket the contribution from faint unseen galaxies. From these two extremes, we predict that the duration of reionization (defined as a change in the mean neutral fraction from 0.9 to 0.1) should be between 2.7 z re 5.7. The large-scale 21 cm power during the advanced EoR stages can be different by up to a factor of ~10, depending on the model. This difference has a comparable contribution from (i) the typical bias of sources and (ii) a more efficient negative feedback in models with an extended EoR driven by faint galaxies. We also present detectability forecasts. With a 1000 h integration, Hydrogen Epoch of Reionization Array and (Square Kilometre Array phase 1) SKA1 should achieve a signal-to-noise of ~few to hundreds throughout the EoR/CD. We caution that our ability to clean foregrounds determines the relative performance of narrow/deep versus wide/shallow surveys expected with SKA1. Our 21-cm power spectra, simulation outputs and visualizations are publicly available.

Description: We use high-resolution N -body simulations to study the concentration and spin parameters of dark matter haloes in the mass range 10 8 M h –1 〈 M 〈 10 11 M h –1 and redshifts 5 〈 z 〈 10, corresponding to the haloes of galaxies thought to be responsible for reionization. We build a subsample of equilibrium haloes and contrast their properties to the full population that also includes unrelaxed systems. Concentrations are calculated by fitting both NFW and Einasto profiles to the spherically averaged density profiles of individual haloes. After removing haloes that are out of equilibrium, we find a z 〉 5 concentration–mass ( c ( M )) relation that is almost flat and well described by a simple power law for both NFW and Einasto fits. The intrinsic scatter around the mean relation is c vir ~ 1 (or 20 per cent) at z = 5. We also find that the analytic model proposed by Ludlow et al. reproduces the mass and redshift dependence of halo concentrations. Our best-fitting Einasto shape parameter, α, depends on peak height, , in a manner that is accurately described by α = 0.0070 2 + 0.1839. The distribution of the spin parameter, , has a weak dependence on equilibrium state; peaks at roughly ~0.033 for our relaxed sample, and at ~0.04 for the full population. The spin–virial mass relation has a mild negative correlation at high redshift.

Description: 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-numerical simulations of the dark ages and the epoch of reionization (EoR), including both X-rays and ultraviolet 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 per cent ionized. (ii) Reionization morphology: a sizeable (~10 per cent) 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, $\bar{x}_{\rm H\,{\small {I}}}$ . Specifically, X-rays produce a dearth of fully neutral regions and a suppression of small-scale ( k 0.1 Mpc –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 0.5 μK 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 sizeable tSZ–dusty galaxy cross-correlation. (iv) The redshifted 21 cm signal: the impact of X-rays on the 21 cm power spectrum during the advanced stages of reionization ( $\bar{x}_{\rm H\,{\small {I}}}\lesssim 0.7$ ) 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 21 cm power spectrum which is ~10–100 times higher at $\bar{x}_{\rm H\,{\small {I}}}\gtrsim 0.9$ 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.

Description: We investigate the impact of sinks of ionizing radiation on the reionization-era 21-cm signal, focusing on one-point statistics. We consider sinks in both the intergalactic medium and inside galaxies. At a fixed filling factor of H ii regions, sinks will have two main effects on the 21-cm morphology: (i) as inhomogeneous absorbers of ionizing photons they result in smaller and more widespread cosmic H ii patches; and (ii) as reservoirs of neutral gas they contribute a non-zero 21-cm signal in otherwise ionized regions. Both effects damp the contrast between neutral and ionized patches during reionization, making detection of the epoch of reionization with 21-cm interferometry more challenging. Here we systematically investigate these effects using the latest seminumerical simulations. We find that sinks dramatically suppress the peak in the redshift evolution of the variance, corresponding to the mid-point of reionization. As previously predicted, skewness changes sign at mid-point, but the fluctuations in the residual H i suppress a late-time rise. Furthermore, large levels of residual H i dramatically alter the evolution of the variance, skewness and power spectrum from that seen at lower levels. In general, the evolution of the large-scale modes provides a better, cleaner, higher signal-to-noise probe of reionization.

Description: We introduce 21 CMMC : a parallelized, Monte Carlo Markov Chain analysis tool, incorporating the epoch of reionization (EoR) seminumerical simulation 21 CMFAST . 21 CMMC estimates astrophysical parameter constraints from 21 cm EoR experiments, accommodating a variety of EoR models, as well as priors on model parameters and the reionization history. To illustrate its utility, we consider two different EoR scenarios, one with a single population of galaxies (with a mass-independent ionizing efficiency) and a second, more general model with two different, feedback-regulated populations (each with mass-dependent ionizing efficiencies). As an example, combining three observations ( z  = 8, 9 and 10) of the 21 cm power spectrum with a conservative noise estimate and uniform model priors, we find that interferometers with specifications like the Low Frequency Array/Hydrogen Epoch of Reionization Array (HERA)/Square Kilometre Array 1 (SKA1) can constrain common reionization parameters: the ionizing efficiency (or similarly the escape fraction), the mean free path of ionizing photons and the log of the minimum virial temperature of star-forming haloes to within 45.3/22.0/16.7, 33.5/18.4/17.8 and 6.3/3.3/2.4 per cent, ~1 fractional uncertainty, respectively. Instead, if we optimistically assume that we can perfectly characterize the EoR modelling uncertainties, we can improve on these constraints by up to a factor of ~few. Similarly, the fractional uncertainty on the average neutral fraction can be constrained to within  10 per cent for HERA and SKA1. By studying the resulting impact on astrophysical constraints, 21 CMMC can be used to optimize (i) interferometer designs; (ii) foreground cleaning algorithms; (iii) observing strategies; (iv) alternative statistics characterizing the 21 cm signal; and (v) synergies with other observational programs.

Description: We use the Fisher matrix formalism and seminumerical simulations to derive quantitative predictions of the constraints that power spectrum measurements on next-generation interferometers, such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA), will place on the characteristics of the X-ray sources that heated the high-redshift intergalactic medium. Incorporating observations between z = 5 and 25, we find that the proposed 331 element HERA and SKA phase 1 will be capable of placing 10 per cent constraints on the spectral properties of these first X-ray sources, even if one is unable to perform measurements within the foreground contaminated ‘wedge’ or the FM band. When accounting for the enhancement in power spectrum amplitude from spin temperature fluctuations, we find that the observable signatures of reionization extend well beyond the peak in the power spectrum usually associated with it. We also find that lower redshift degeneracies between the signatures of heating and reionization physics lead to errors on reionization parameters that are significantly greater than previously predicted. Observations over the heating epoch are able to break these degeneracies and improve our constraints considerably. For these two reasons, 21-cm observations during the heating epoch significantly enhance our understanding of reionization as well.