Foregrounds for redshifted 21-cm studies of reionization: Giant Meter Wave Radio Telescope 153-MHz observations

Monthly Notices of the Royal Astronomical Society, Apr 2008

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 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 model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ(U, Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ(U, Δν), 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.

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Foregrounds for redshifted 21-cm studies of reionization: Giant Meter Wave Radio Telescope 153-MHz observations

C 2008 The Authors. Journal compilation C 2008 RAS Foregrounds for redshifted 21-cm studies of reionization: Giant Meter Wave Radio Telescope 153-MHz observations Sk. Saiyad Ali 1 Somnath Bharadwaj 1 Jayaram N. Chengalur 0 0 National Centre for Radio Astrophysics , TIFR, Post Bag 3, Ganeshkhind, Pune 411 007 , India 1 Department of Physics and Meteorology & Centre for Theoretical Studies, IIT Kharagpur , 721 302 , India A B S T R A C T 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 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 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. methods; statistical - cosmology; observations - diffuse radiation 1 I N T R O D U C T I O N Observations of redshifted 21-cm radiation from the large-scale distribution of neutral hydrogen (H I) are perceived as one of the most promising future probes of the Universe at high redshifts (see Furlanetto, Oh & Briggs 2006, for a recent review). Observational evidence from quasar absorption spectra (Becker et al. 2001; Fan et al. 2002) and the Cosmic Microwave Background Radiation (Page et al. 2007; Spergel et al. 2007) together imply that the H I was reionized over an extended period spanning the redshift range 6 z 15 (for reviews see Barkana & Loeb 2001; Choudhury & Ferrara 2006; Fan, Carilli & Keating 2006). Determining how and when the Universe was reionized is one of the most important issue that will be addressed by future 21-cm observations. The Giant Meter Wave Radio Telescope (GMRT;1 Swarup et al. 1991) currently functioning at several frequency bands in the range 150–1420 MHz is very well suited for carrying out initial investigations towards detecting the reionization H I signal. There are several upcoming low-frequency instruments such as LOFAR,2 MWA,3 21CMA4 and SKA5 which are being built specifically with these observations in view. It is currently perceived that a statistical analysis of the fluctuations in the redshifted 21-cm signal holds the greatest potential for observing H I at high redshifts (Bharadwaj & Sethi 2001; Morales & Hewitt 2004; Zaldarriaga, Furlanetto & Hernquist 2004; Bharadwaj & Ali 2005; Bharadwaj & Pandey 2005). Correlations among the visibilities measured in radio-interferometric observations directly probe the H I power spectrum at the epoch where the radiation originated. The reionization visibility signal at the GMRT is expected to be ∼1 mJy and smaller (Bharadwaj & Ali 2005). This H I signal is present as a minute component of the background in all low-frequency observations, and it is buried in foreground radiation from other astrophysical sources whose contribution is four to five orders of magnitude larger. Extracting the H I signal from the foregrounds is a major challenge. Individual sources can be identified and removed from the image at a flux level which depends on the sensitivity. The contribution from the remaining discrete sources could be large enough to overwhelm the H I signal (Di Matteo et al. 2002). The diffuse synchrotron E-mail: (SSA); (SB); (JNC) 1 http://www.gmrt.ncra.tifr.res.in 2 http://www.lofar.org/ 3 http://www.haystack.mit.edu/arrays/MWA 4 http://web.phys.cmu.edu/∼past/ 5 http://www.skatelescope.org/ emission from our Galaxy (Shaver et al. 1999) is another important component. Foreground sources include free–free emission from ionizing haloes (Oh & Mack 2003), faint radio-loud quasars (Di Matteo et al. 2002) and synchrotron emission from low-redshift galaxy clusters (Di Matteo, Ciardi & Miniati 2004). The foregrounds are expected to have a continuum spectra, and the contribution at two different frequencies separated by ν ∼ 1 MHz is expected to be highly correlated. The H I signal is expected to be uncorrelated at such a frequency separation and this holds the promise of allowing us t (...truncated)


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Sk. Saiyad Ali, Somnath Bharadwaj, Jayaram N. Chengalur. Foregrounds for redshifted 21-cm studies of reionization: Giant Meter Wave Radio Telescope 153-MHz observations, Monthly Notices of the Royal Astronomical Society, 2008, pp. 2166-2174, 385/4, DOI: 10.1111/j.1365-2966.2008.12984.x