Foregrounds for observations of the cosmological 21 cm line I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field
Foregrounds for observations of the cosmological 21 cm line I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field
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DOI：
10.1051/0004-6361/200911627
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发表时间：
2009-06-01
影响因子：
6.5
通讯作者：
Zaroubi, S.
中科院分区：
文献类型：
--
作者：
Bernardi, G.;de Bruyn, A. G.;Zaroubi, S.
上下文宇宙学的21厘米线有望成为宇宙学的强大工具，允许调查所谓的黑暗时代的结束，当第一个星系形成时。天体物理前景预计比宇宙学信号大大约三个数量级，因此代表了宇宙学21厘米线的严重污染。详细了解它们的强度和偏振结构的相关角标度的1-30 arcmin将是必不可少的宇宙学信号从数据中提取。我们提出的第一个结果，从一系列的观测与Westerbork望远镜在140-160 MHz的范围内，2弧分的分辨率，旨在表征时代的再电离实验的前景的属性。利用旋转测度综合技术对偏振数据进行了分析。计算了总光强和偏振角功率谱。我们第一次在150兆赫频率上探测到银河系弥散辐射的波动，其尺度大于13弧分，位于银河系低纬度地区，称为扇形区，中心在α = 3（h）10（m），δ = 65度30 '。这些波动的均方根为14 K。总强度功率谱示出了下至l的幂律行为，其类似于900，斜率β（I）（l）= -2.2 +/-0.3。在较小的角度尺度上的漫发射的检测受到残余点源的限制。我们测量到了与3 mJy束（-1）相近的均方根混淆噪声，并首次在此频率下探测到了漫偏振辐射。极化信号在空间和沿着视线方向上都表现出复杂的结构。偏振功率谱不受剩余点源的影响，只受热噪声的限制。它显示出类似于2700的幂律行为，斜率beta（P）（l）= -1.65 +/- 0.15。在4个弧分尺度上，偏振涨落的均方根为7.2 K。测量的总强度波动被用来估计前景污染的宇宙学信号。通过外推总强度辐射谱，我们发现在5个角分尺度上，Δ T = rootl（1 + 1）C-1（I）/2 pi的污染值近似于5.7K，在同一角尺度上，对应的均方根值近似于18.3K，偏振功率谱的能级近似于3.3K。然而，扇形区域不能代表其他天空区域，因为它的偏振信号异常明亮，但很可能代表银河系中纬度和高纬度天空亮度的上限。
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.