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...

We compute the spherically-averaged power spectrum from four seasons of data obtained for the Epoch of Reionisation (EoR) project observed with the Murchison Widefield Array (MWA). We measure the EoR power spectrum over k = 0.07 − 3.0 hMpc −1 at redshifts z = 6.5 − 8.7. The largest aggregation of 110 hours on EoR0 high-band (3,340 observations), yields a lowest measurement of (43 mK) 2 = 1.8×10 3 mK 2 at k=0.14 hMpc −1 and z = 6.5 (2σ thermal noise plus sample variance). Using the Real-Time System to calibrate and the CHIPS pipeline to estimate power spectra, we select the best observations from the central five pointings within the 2013-2016 observing seasons, observing three independent fields and in two frequency bands. This yields 13,591 2-minute snapshots (453 hours), based on a quality assurance metric that measures ionospheric activity. We perform another cut to remove poorly-calibrated data, based on power in the foreground-dominated and EoR-dominated regions of the twodimensional power spectrum, reducing the set to 12,569 observations (419 hours). These data are processed in groups of 20 observations, to retain the capacity to identify poor data, and used to analyse the evolution and structure of the data over field, frequency, and data quality. We subsequently choose the cleanest 8,935 observations (298 hours of data) to form integrated power spectra over the different fields, pointings and redshift ranges.