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)