GMRT observation towards detecting the post-reionization 21-cm signal

Abstract

The redshifted 21-cm signal from neutral hydrogen (H I) is an important future probe of the high-redshift Universe. We have analysed 610 MHz Giant Metrewave Radio Telescope (GMRT) observations towards detecting this signal from z= 1.32. The multi-frequency angular power spectrum C_l(Δν) is used to characterize the statistical properties of the background radiation across angular scales ^~20 arcsec to 10 arcmin, and a frequency bandwidth of 7.5 MHz with resolution 125 kHz. The measured C_l(Δν) which ranges from 7 to 18 mK² is dominated by foregrounds, the expected H I signal C^HI_l (Δν) ~ 10⁻⁶ to 10⁻⁷ mK² is several orders of magnitude smaller and detecting this is a big challenge. The foregrounds, believed to originate from continuum sources, is expected to vary smoothly with Δν whereas the H I signal decorrelates within ^~0.5 MHz, and this holds the promise of separating the two. For each l, we use the interval 0.5 ≤Δν≤ 7.5 MHz to fit a fourth-order polynomial which is subtracted from the measured Cl(Δν) to remove any smoothly varying component across the entire bandwidth Δν≤ 7.5 MHz. The residual C_l(Δν), we find, has an oscillatory pattern with amplitude and period, respectively, ^~0.1 mK² and Δν= 3 MHz at the smallest l value of 1476, and the amplitude and period decreasing with increasing l. Applying a suitably chosen high pass filter, we are able to remove the residual oscillatory pattern for l= 1476 where the residual C_l(Δν) is now consistent with zero at the 3σ noise level. Based on this we conclude that we have successfully removed the foregrounds at l= 1476 and the residuals are consistent with noise. We use this to place an upper limit on the H I signal whose amplitude is determined by ̅X_HIb(C^HI_l(Δν) α [̅x_HIb]², where and b are the H I neutral fraction and the H I bias, respectively. A value of greater than 7.95 would have been detected in our observation, and is therefore ruled out at the 3σ level. For comparison, studies of quasar absorption spectra indicate ̅x_HI ~ 2.5 × 10⁻² which is ^~ 330 times smaller than our upper limit. We have not succeeded in completely removing the residual oscillatory pattern, whose cause is presently unknown to us, for the larger l values.