Improved foreground removal in GMRT 610 MHz observations towards redshifted 21-cm tomography

TL;DR

This paper demonstrates an improved method for foreground removal in GMRT 610 MHz observations, reducing oscillatory patterns in the data to better isolate the 21-cm signal from the high redshift Universe.

Contribution

It introduces sidelobe suppression of the primary antenna elements combined with polynomial fitting to effectively reduce foreground oscillations in 21-cm tomography data.

Findings

Oscillatory patterns along  in the power spectrum were significantly reduced.

Residuals after suppression and fitting are 0 mK^2, consistent with noise.

The method sets a 99% upper limit on the HI signal with xHI b  2.9.

Abstract

Foreground removal is a challenge for 21-cm tomography of the high redshift Universe. We use archival GMRT data (obtained for completely different astronomical goals) to estimate the foregrounds at a redshift ~ 1. The statistic we use is the cross power spectrum between two frequencies separated by \Delta{\nu} at the angular multipole l, or equivalently the multi-frequency angular power spectrum C_l(\Delta{\nu}). An earlier measurement of C_l(\Delta{\nu}) using this data had revealed the presence of oscillatory patterns along \Delta{\nu}, which turned out to be a severe impediment for foreground removal (Ghosh et al. 2011). 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 (\leq 0.02 mK^2), consistent with the noise at the 3{\sigma} level. Since the polynomial fitting is done after estimation of the power spectrum it can be ensured that the estimation of the HI signal is not biased. The corresponding 99% upper limit on the HI signal is xHI b \leq 2.9, where xHI is the mean neutral fraction and b is the bias.