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Characterizing foreground for redshifted 21 cm radiation: 150 MHz Giant Metrewave Radio Telescope observations

Authors

  • Abhik Ghosh,

    Corresponding author
    • Department of Physics and Meteorology and Centre for Theoretical Studies, IIT Kharagpur, Kharagpur, India
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  • Jayanti Prasad,

    Corresponding author
    1. IUCAA, Ganeshkhind, Pune 411 007, India
    • Department of Physics and Meteorology and Centre for Theoretical Studies, IIT Kharagpur, Kharagpur, India
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  • Somnath Bharadwaj,

    Corresponding author
    • Department of Physics and Meteorology and Centre for Theoretical Studies, IIT Kharagpur, Kharagpur, India
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  • Sk. Saiyad Ali,

    Corresponding author
    1. Department of Physics, Jadavpur University, Kolkata, India
    • Department of Physics and Meteorology and Centre for Theoretical Studies, IIT Kharagpur, Kharagpur, India
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  • Jayaram N. Chengalur

    Corresponding author
    1. National Centre for Radio Astrophysics, TIFR, Ganeshkhind, Pune 411 007, India
    • Department of Physics and Meteorology and Centre for Theoretical Studies, IIT Kharagpur, Kharagpur, India
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E-mail: abhik@phy.iitkgp.ernet.in (AG); jayanti@iucaa.ernet.in (JP); somnath@phy.iitkgp.ernet.in (SB); saiyad@phys.jdvu.ac.in (SSA); chengalu@ncra.tifr.res.in (JNC)

ABSTRACT

Foreground removal is a major challenge for detecting the redshifted 21 cm neutral hydrogen (H i) signal from the Epoch of Reionization. We have used 150 MHz Giant Metrewave Radio Telescope observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multifrequency angular power spectrum C(Δν) is found to have values in the range 104–2 × 104 mK2 across 700 ≤ ℓ ≤ 2 × 104 and Δν ≤ 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured C(Δν) does not show a smooth Δν dependence, which poses a severe difficulty for foreground removal using polynomial fitting.

The observational data were used to assess point source subtraction. Considering the brightest source (∼1 Jy) in each field, we find that the residual artefacts are less than 1.5 per cent in the most sensitive field (FIELD I). Considering all the sources in the fields, we find that the bulk of the image is free of artefacts, the artefacts being localized to the vicinity of the brightest sources. We have used FIELD I, which has an rms noise of 1.3 mJy beam−1, to study the properties of the radio source population to a limiting flux of 9 mJy. The differential source count is well fitted with a single power law of slope −1.6. We find there is no evidence for flattening of the source counts towards lower flux densities which suggests that source population is dominated by the classical radio-loud active galactic nucleus.

The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I. We find C ∝ ℓ−2.34 for 253 ≤ ℓ ≤ 800 which is characteristic of the Galactic synchrotron radiation measured at higher frequencies and larger angular scales. We estimate the fluctuations in the Galactic synchrotron emission to be inline image at ℓ = 800 (θ > 10 arcmin). The measured C is dominated by the residual point sources and artefacts at smaller angular scales where C ∼ 103 mK2 for ℓ > 800.

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