Abstract
One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 hours of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum, and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.
Highlights
Studying 21-cm radiation from hydrogen at high redshifts (Field 1958, 1959; Hogan & Rees 1979; Scott & Rees 1990; Kumar, Subramanian & Padmanabhan 1995; Madau, Meiksin & Rees 1997) promises to be interesting for several reasons
The quality of extraction is affected by several factors: the observational strategy and the length of observations, which affect the volume being studied and the level of thermal noise; the array design and layout; the foregrounds from Galactic and extragalactic sources, and the methods used to remove their influence from the data; excision of radio-frequency interference (RFI) and radio recombination lines; and, for example, the quality of polarization and total intensity calibration for instrumental and ionospheric effects
In this paper we have studied the extraction of the 21-cm Epoch of Reionization (EoR) power spectrum from simulated Low Frequency Array (LOFAR) data
Summary
Studying 21-cm radiation from hydrogen at high redshifts (Field 1958, 1959; Hogan & Rees 1979; Scott & Rees 1990; Kumar, Subramanian & Padmanabhan 1995; Madau, Meiksin & Rees 1997) promises to be interesting for several reasons. Fluctuations in intensity are sourced partly by density fluctuations, measurements of which may allow rather tight constraints on cosmological parameters (Mao et al 2008). The quality of extraction is affected by several factors: the observational strategy and the length of observations, which affect the volume being studied and the level of thermal noise; the array design and layout; the foregrounds from Galactic and extragalactic sources, and the methods used to remove their influence from the data (presumably by exploiting their assumed smoothness as a function of frequency; see e.g. Shaver et al 1999; Di Matteo et al 2002; Oh & Mack 2003; Zaldarriaga, Furlanetto & Hernquist 2004); excision of radio-frequency interference (RFI) and radio recombination lines; and, for example, the quality of polarization and total intensity calibration for instrumental and ionospheric effects. We generate data cubes realistic enough so that we can test different observing strategies and methods of subtracting the foregrounds, and look at the effect on the inferred power spectrum.
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