Exploring the low-frequency Universe with the MWA.

Epoch of Reionisation

Exploration of the Cosmic Dawn, the period when the first stars and galaxies formed in the first few hundred million years after the Big Bang, has been identified as an important area for new discoveries within the next decade. In the period following, referred to as the Epoch of Reionisation (EoR), ionising light from these sources transformed the universe, ionising the neutral gas filling the intergalactic space, composed predominantly of atomic hydrogen. The MWA is one of the first radio interferometers to attempt to detect redshifted 21 cm line emission from neutral hydrogen gas in the intergalactic medium during this period.

The opaque universe

The EoR refers to the period in the history of the universe during which the predominantly neutral intergalactic medium was ionised by high-energy radiation from the first luminous sources. These sources may have been stars, galaxies, quasars, or some combination of the above. By studying reionisation, we can learn a great deal about the process of structure formation in the universe, and find the evolutionary links between the remarkably smooth matter distribution at early times revealed by CMB studies, and the highly structured universe of galaxies and clusters of galaxies at redshifts of six and below.

The MWA is designed to provide detailed information about conditions in the intergalactic medium during and immediately preceding the EoR. In particular, Phase II with its two hexagonal sub-arrays with 72 tiles in a regular configuration, is designed to provide the precise calibration and high sensitivity required for this challenging experiment.

Detecting and Measuring the Statistical EOR Signal

The MWA does not have sufficient sensitivity to directly image individual features in the EoR hydrogen signal. That capability must await a manyfold increase in physical collecting area, and is expected to be realised by the Square Kilometre Array by the end of the 2020s. The MWA’s lack of collecting area can, however, be effectively compensated for by increasing the effective field of view of the instrument. By measuring the intensity at many locations of the sky (and at many locations along the line of sight, by using different frequencies corresponding to different redshifts), we can obtain a statistical measure of the fluctuations even when the signal is too weak to see in any given pixel. This power spectrum measurement is similar to the type of measurement done by microwave background experiments, except that for the EoR we also measure fluctuations along the line-of-sight, rather than the angular spectrum of the CMB. MWA has been designed to yield a high fidelity measurement of the EoR power spectrum shape using a few hundred hours of observing time.

Foreground Subtraction, Avoidance and Treatment

A major challenge for any EoR experiment is the fact that the signals being sought are far fainter than other types of emission from the sky. Treating these troublesome foregrounds is a principal focus of instrument design and data analysis techniques. Fortunately, many of the strongest foregrounds have properties that are easily separable from the EoR signals, and one of the strongest discriminants is that most foregrounds are spectrally smooth. By contrast, the EoR signals are expected to show strong fluctuations over small ranges of frequency. One of the most difficult foregrounds to deal with is polarized emission from our own Galaxy. While it is intrinsically smooth, the clumpy interstellar medium imprints spatial and frequency structures on it which will be difficult to remove. A high precision polarimetric calibration is required, and has been an important design driver for the array.

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