Concerted effort is currently ongoing to open up the Epoch of Reionization (z ∼15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-α emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from z ∼6-200 for astrophysical and cosmological studies, opening not only the Epoch of Reionization, but also its preceding Cosmic Dawn (z ∼30-15) and possibly even the later phases of the Dark Ages (z ∼200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the Epoch of Reionization, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array (SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range z ∼6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume. This is done within the framework of the current SKA1 baseline design and a nominal CD/EoR straw-man survey, consisting of a shallow, medium-deep and deep survey, the latter probing down to ∼1 mK brightness temperature on arc-minute scales at the end of reionization. Possible minor modifications to the design of SKA1 and the upgrade to SKA2 are discussed, in addition to science that could be done already during roll-out when SKA1 still has limited capabilities and/or core collecting area.Advancing Astrophysics with the Square Kilometre Array

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We use the latest multi-redshift (z = 6.5 − 8.7) upper limits on the 21-cm signal from the Murchison Widefield Array (MWA) to explore astrophysical models which are inconsistent with the data. We explore these limits in the context of reionisation astrophysics by using 21CMMC to connect the disfavoured regions of parameter space to existing observational constraints on reionisation such as high-z galaxy ultra-violet (UV) luminosity functions, the background UV photoionisation rate, the intergalactic medium (IGM) neutral fraction, the electron scattering optical depth and the soft-band X-ray emissivity. We find the vast majority of disfavoured models to already be inconsistent with existing observational constraints. These can be broadly classified into two types of models: (i) ‘cold’ reionisation and (ii) pure matter density fluctuations in a cold, neutral IGM (i.e. no reionisation). Interestingly, a small subsample of models inconsistent with the MWA are consistent with the aforementioned constraints (excluding the X-ray emissivity). This implies that the current MWA limits are already providing unique information to disfavour models of reionisation, albeit extremely weakly. We also provide the first limits on the soft-band X-ray emissivity from galaxies at high redshifts, finding 1σ lower limits of εX, 0.5 − 2 keV ≳ 1034.5 erg s−1 Mpc−3. Finally, we recover 95 per cent disfavoured limits on the IGM spin temperature of $\bar{T}_{\rm S}\lesssim$ 1.3, 1.4, 1.5, 1.8, 2.1, 2.4 K at z = 6.5, 6.8, 7.1, 7.8, 8.2, 8.7. With this we infer the IGM must have undergone, at the very least, a small amount of X-ray heating. Note, the limits on εX, 0.5 − 2 keV and $\bar{T}_{\rm S}$ are conditional on the IGM neutral fraction.

To make a power spectrum (PS) detection of the 21-cm signal from the Epoch of Reionisation (EoR), one must avoid/subtract bright foreground sources. Sources such as Fornax A present a modelling challenge due to spatial structures spanning from arc seconds up to a degree. We compare modelling with multi-scale (MS) CLEAN components to ‘shapelets’, an alternative set of basis functions. We introduce a new image-based shapelet modelling package, SHAMFI. We also introduce a new CUDA simulation code (WODEN) to generate point source, Gaussian, and shapelet components into visibilities. We test performance by modelling a simulation of Fornax A, peeling the model from simulated visibilities, and producing a residual PS. We find the shapelet method consistently subtracts large-angular-scale emission well, even when the angular resolution of the data is changed. We find that when increasing the angular resolution of the data, the MS CLEAN model worsens at large angular scales. When testing on real Murchison Widefield Array data, the expected improvement is not seen in real data because of the other dominating systematics still present. Through further simulation, we find the expected differences to be lower than obtainable through current processing pipelines. We conclude shapelets are worthwhile for subtracting extended galaxies, and may prove essential for an EoR detection in the future, once other systematics have been addressed.