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

Abstract

Foreground removal is a major challenge for detecting the redshifted 21cm neutral hydrogen (Hi) signal from the Epoch of Reionization. We have used 150MHz 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 Cℓ(Δν) is found to have values in the range 104-2 × 104mK2 across 700 ≤ ℓ ≤ 2 × 104 and Δν ≤ 2.5MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured Cℓ(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 (∼1Jy) 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 1.3mJybeam-1, to study the properties of the radio source population to a limiting flux of 9mJy. 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 Cℓ ∝ 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 ℓ(ℓ+1)Cℓ/2π≃10K at ℓ = 800 (θ > 10arcmin). The measured Cℓ is dominated by the residual point sources and artefacts at smaller angular scales where Cℓ ∼ 103mK2 for ℓ > 800. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.