Foregrounds for observations of the cosmological 21 cm line - I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field

By G. Bernardi, D. Bruyn, M. Brentjens, Benedetta Ciardi, G. Harker, V. Jelic, Leon V. E. Koopmans, P. Labropoulos, A. Offringa, V. Pandey and 4 more. 2009

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Context. The cosmological 21 cm line promises to be a formidable tool for cosmology, allowing the investigation of the end of the so-called dark ages, when the first galaxies formed. Aims. Astrophysical foregrounds are expected to be about three orders of magnitude greater than the cosmological signal and therefore represent a serious contamination of the cosmological 21 cm line. Detailed knowledge of both their intensity and polarization structure on the relevant angular scale of 1-30 arcmin will be essential for extracting the cosmological signal from the data. Methods. We present the first results from a series of observations conducted with the Westerbork telescope in the 140-160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments. The polarization data were analysed through the rotation measure synthesis technique. We computed total intensity and polarization angular power spectra. Results. For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low Galactic latitude area known as Fan region, centred at alpha = 3(h)10(m), delta = 65 degrees 30'. Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power-law behaviour down to l similar to 900 with slope beta(I)(l) = -2.2 +/- 0.3. The detection of diffuse emission at smaller angular scales is limited by residual point sources. We measured an rms confusion noise of similar to 3 mJy beam(-1). Diffuse polarized emission was also detected for the first time at this frequency. The polarized signal shows complex structure both spatially and along the line of sight. The polarization power spectrum is not affected by residual point sources and is only limited by the thermal noise. It shows a power-law behaviour down to l similar to 2700 with slope beta(P)(l) = -1.65 +/- 0.15. The rms of polarization fluctuations is 7.2 K on 4 arcmin scales. Conclusions. The measured total intensity fluctuations are used to estimate the foreground contamination on the cosmological signal. By extrapolating the spectrum of total intensity emission, we find a contamination of delta T = root l(l + 1)C(l)(I)/2 pi similar to 5.7 K on 5 arcmin scales and a corresponding rms value of similar to 18.3 K at the same angular scale. The level of the polarization power spectrum is delta T similar to 3.3 K on 5 arcmin scales. However, the Fan region cannot be taken as representative of other sky regions, given its exceptionally bright polarized signal, but is likely to represent an upper limit on the sky brightness at moderate and high Galactic latitude.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

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Citations in the corpus

Reionization and Cosmology with 21-cm Fluctuations (2009, background): “Faraday rotation by the interstellar magnetic field creates a f−2 rotation of the observed polarized Galactic synchrotron emission (φ ∝ λ2, de Bruyn et al. 2006, Schnitzeler, Katgert & de Bruyn 2009, Bernardi et al. 2009, Pen et al. 2008a).”
Science with the Murchison Widefield Array (2012, background): “…inner Galactic plane at 1.4 GHz (Reich et al. 1990; Wolleben et al. 2006; Haverkorn et al. 2006) and it is likely that much of the inner Galactic plane will appear completely depolarised below 300 MHz, although Bernardi et al. (2009) have detected polarized emission at arcminute scales at 150 MHz.”
The GMRT Epoch of Reionization experiment: a new upper limit on the neutral hydrogen power spectrum at z≈ 8.6 (2010, result): “Bernardi et al. (2009) reported a power spectrum of foregrounds without subtraction in the galactic plane at a level comparable to our measurement both here and in Pen et al. (2009).”
PAPER-64 CONSTRAINTS ON REIONIZATION: THE 21 cm POWER SPECTRUM AT z = 8.4 (2015, background): “…where 21 cm reionization experiments operate, emission from these foregrounds is brighter than the predicted reionization signal by several orders of magnitude (Santos et al. 2005; Ali et al. 2008; de Oliveira-Costa et al. 2008; Jelić et al. 2008; Bernardi et al. 2009, 2010; Ghosh et al. 2011).”
Hydrogen Epoch of Reionization Array (HERA) (2016, background): “These experiments all struggle with the inherent challenge of simultaneously meeting stringent sensitivity requirements while suppressing foregrounds ∼ 5− 6 orders of magnitude brighter than the 21 cm signal (Bernardi et al. 2009; Pober et al. 2013a; Dillon et al. 2014).”
An improved source-subtracted and destriped 408 MHz all-sky map (2014, background): “However, we note that recent higher resolution observations suggest that the spectrum is flatter on smaller angular scales (` & 1000; Bernardi et al. 2009).”
Reionization and the Cosmic Dawn with the Square Kilometre Array (2012, methodology): “…most recent and comprehensive targeted observations of the foregrounds have been done by the LOFAR-EoR team, using the Westerbork Radio Synthesis Telescope (WSRT; Bernardi et al. 2009, 2010) and the Low Frequency Array (LOFAR; Jelic et al., Labropoulos et al., and Yattawata et al., in preparation).”