Foregrounds for redshifted 21 cm studies of reionization : GMRT 153 MHz observations

Foreground subtraction is the biggest challenge for future redshifted 21 cm observations to probe reionization. We use a short GMRT observation at 153MHz to characterize the statistical properties of the background radiation across ∼ 1◦ to sub-arcminutes angular scales, and across a frequency band of 5MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum Cl. 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 V2(U, 0). From our observations we find that V2(U, 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the 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% variation across 2.5MHz. In this case however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures.