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F. H. Briggs (ANU/ATNF)
The observation of the 21cm line at the redshift of the Epoch of Reionization (EoR) will be an exercise in separating weak, diffuse, thermal emission emanating from primordial hydrogen from bright, steep spectrum, non-thermal foregrounds. The precise redshifts at which the EoR proves to be most interesting will dictate the principal frequency bands for observation, but, even under favourable conditions, the predictions have 21cm line emission a factor ~104 below the brightness of the radio sky.
Some factors enter in favour of the observation: The signal has distinct spectral character on frequency scales of a few MHz; this serves to separate it from the strongest foregrounds, which have smooth, power law spectra over much wider ranges. The locations and properties of the brightest discrete sources of competing emssion are known and catalogued, so to first order, they become a contributor to calibration schemes and eventually are subtracted.
From a practical point of view, instrumentation for low frequencies is inexpensive, since telescopes can be electronically-steered arrays built of wires and mesh with uncooled amplifiers. Spectral line widths from high redshift features are compressed by factor (1+z) in frequency, so physical structures in redshift space are a good match to the bandwidths that are achievable with commercially available communications and signal processing devices.
The first generation of EoR telescopes are tuned to probe angular scales of 1 to ~30 arcminutes. These arrays concentrate their collecting area within a ~0.5 km diameter area in order to achieve high surface brightness sensitivity. Even so,the first step will likely be a statistical detection of the power spectrum of brightness fluctuation as a function of redshift.
Subtle chromatic effects in array design and radio frequency interference RFI are among the technical hurdles. Fortunately, technical advances in signal processing and rfi cancellation can be brought to bear.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.