Foreground removal is a major challenge for detecting the redshifted 21 cm neutral hydrogen (H I) signal from the Epoch of Reionization. We have used 150 MHz Giant Metrewave Radio Telescope observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multifrequency angular power spectrum C ( ν) is found to have values in the range 10 4 -2 × 10 4 mK 2 across 700 ≤ ≤ 2 × 10 4 and ν ≤ 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured C ( ν) does not show a smooth ν dependence, which poses a severe difficulty for foreground removal using polynomial fitting.The observational data were used to assess point source subtraction. Considering the brightest source (∼1 Jy) in each field, we find that the residual artefacts are less than 1.5 per cent in the most sensitive field (FIELD I). Considering all the sources in the fields, we find that the bulk of the image is free of artefacts, the artefacts being localized to the vicinity of the brightest sources. We have used FIELD I, which has an rms noise of 1.3 mJy beam −1 , to study the properties of the radio source population to a limiting flux of 9 mJy. The differential source count is well fitted with a single power law of slope −1.6. We find there is no evidence for flattening of the source counts towards lower flux densities which suggests that source population is dominated by the classical radio-loud active galactic nucleus.The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I. We find C ∝ −2.34 for 253 ≤ ≤ 800 which is characteristic of the Galactic synchrotron radiation measured at higher frequencies and larger angular scales. We estimate the fluctuations in the Galactic synchrotron emission to be √ ( + 1)C /2π

Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1 • to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum C l . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V 2 (U, 0). From our observations, we find that V 2 (U, 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ(U, ν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ (U, ν), in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures.

We present two estimators to quantify the angular power spectrum of the sky signal directly from the visibilities measured in radio interferometric observations. This is relevant for both the foregrounds and the cosmological 21-cm signal buried therein. The discussion here is restricted to the Galactic synchrotron radiation, the most dominant foreground component after point source removal. Our theoretical analysis is validated using simulations at 150 MHz, mainly for GMRT and also briefly for LO-FAR. The Bare Estimator uses pairwise correlations of the measured visibilities, while the Tapered Gridded Estimator uses the visibilities after gridding in the uv plane. The former is very precise, but computationally expensive for large data. The latter has a lower precision, but takes less computation time which is proportional to the data volume. The latter also allows tapering of the sky response leading to sidelobe suppression, an useful ingredient for foreground removal. Both estimators avoid the positive bias that arises due to the system noise. We consider amplitude and phase errors of the gain, and the w-term as possible sources of errors . We find that the estimated angular power spectrum is exponentially sensitive to the variance of the phase errors but insensitive to amplitude errors. The statistical uncertainties of the estimators are affected by both amplitude and phase errors. The w-term does not have a significant effect at the angular scales of our interest. We propose the Tapered Gridded Estimator as an effective tool to observationally quantify both foregrounds and the cosmological 21-cm signal.

Redshifted 21-cm radiation originating from the cosmological distribution of neutral hydrogen (H I) appears as background radiation in low-frequency radio observations. The angular and frequency domain fluctuations in this radiation carry information concerning cosmological structure formation. We propose that correlations between visibilities measured at different baselines and frequencies in radio-interferometric observations be used to quantify the statistical properties of these fluctuations. This has an inherent advantage over other statistical estimators in that it deals directly with the visibilities which are the primary quantities measured in radio-interferometric observations. Also, the visibility correlation has a very simple relation with the power spectrum. We present estimates of the expected signal for nearly the entire post-recombination era, from the dark ages to the present epoch. The epoch of reionization, where H I has a patchy distribution, has a distinct signature where the signal is determined by the size of the discrete ionized regions. The signal at other epochs, where H I follows the dark matter, is determined largely by the power spectrum of dark matter fluctuations. The signal is strongest for baselines where the antenna separations are within a few hundred times the wavelength of observation, and an optimal strategy would preferentially sample these baselines. In the frequency domain, for most baselines the visibilities at two different frequencies are uncorrelated beyond ν ∼ 1 MHz, a signature which, in principle, would allow the H I signal to be easily distinguished from the continuum sources of contamination.

Foreground removal is a challenge for 21‐cm tomography of the high‐redshift Universe. We use archival Giant Metrewave Radio Telescope (GMRT) data (obtained for completely different astronomical goals) to estimate the foregrounds at a redshift of ∼1. The statistic we use is the cross power spectrum between two frequencies separated by Δν at the angular multipole ℓ, or equivalently the multi‐frequency angular power spectrum Cℓ(Δν). An earlier measurement of Cℓ(Δν) using these data had revealed the presence of oscillatory patterns along Δν, which turned out to be a severe impediment for foreground removal. Using the same data, in this paper we show that it is possible to considerably reduce these oscillations by suppressing the sidelobe response of the primary antenna elements. The suppression works best at the angular multipoles ℓ for which there is a dense sampling of the u−v plane. For three angular multipoles ℓ= 1405, 1602 and 1876, this sidelobe suppression along with a low order polynomial fitting completely results in residuals of (≤ 0.02 mK2), consistent with the noise at the 3σ level. Since the polynomial fitting is done after estimation of the power spectrum it can be ensured that the estimation of the H i signal is not biased. The corresponding 99 per cent upper limit on the H i signal is , where is the mean neutral fraction and b is the bias.