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Characterizing Foreground for redshifted 21-cm radiation: 150 MHz GMRT observations

Foreground removal is a major challenge for detecting the redshifted 21-cm neutral hydrogen (HI) signal from the Epoch of Reionization (EoR). We have used 150 MHz GMRT observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multi-frequency angular power spectrum C_l(Delta nu) is found to have values in the range 10^4 mK^2 to 2 x 10^4 mK^2 across 700 <= l <= 2 x 10^4 and Delta nu <= 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured C_l(Delta nu) does not show a smooth Delta nu dependence, which poses a severe difficulty for foreground removal using polynomial fitting. The observational data was used to assess point source subtraction. Considering the brightest source (~ 1 Jy) in each field, we find that the residual artifacts are less than 1.5% in the most sensitive field (FIELD I). We have used FIELD I, which has a rms noise of 1.3 mJy/Beam, 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 (AGN). The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I . We find C_l \propto 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 sqrt{l(l+1)C_l/2 pi} ~ 10 K at l=800 (theta > 10'). The measured C_l is dominated by the residual point sources and artifacts at smaller angular scales where C_l ~ 10^3 mK^2 for l > 800.