GMRT observation towards detecting the postreionization 21cm signal
Abstract
The redshifted 21cm signal from neutral hydrogen (H I) is an important future probe of the highredshift Universe. We have analysed 610 MHz Giant Metrewave Radio Telescope (GMRT) observations towards detecting this signal from z= 1.32. The multifrequency 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 mK^{2} is dominated by foregrounds, the expected H I signal C^{HI}_{ℓ} (Δν) ∼ 10^{6} to 10^{7} mK^{2} 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 fourthorder polynomial which is subtracted from the measured C_{ℓ}(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 mK^{2} 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 barχ_{H1}b(C^{H1}_{ell;}(Δν)∝[barχ_{H1}b]^{2}, where barχ_{H1} and b are the H I neutral fraction and the H I bias, respectively. A value of barχ_{H1}b 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 barχ_{H1} ≈ 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.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 March 2011
 DOI:
 10.1111/j.13652966.2010.17853.x
 arXiv:
 arXiv:1010.4489
 Bibcode:
 2011MNRAS.411.2426G
 Keywords:

 cosmology: observations;
 diffuse radiation;
 largescale structure of Universe;
 Astrophysics  Cosmology and Extragalactic Astrophysics
 EPrint:
 29 pages, 13 figures, Accepted to MNRAS