Radio foregrounds for the 21 centimeter tomography of the neutral intergalactic medium at high redshifts

Absorption or emission against the cosmic microwave background (CMB) radiation may be observed in the redshifted 21 cm line if the spin temperature of the neutral intergalactic medium (IGM) prior to reionization differs from the CMB temperature. This so-called 21 cm tomography should reveal important information on the physical state of the IGM at high redshifts. The fluctuations in the redshifted 21 cm line, due to gas density inhomogeneities at early times, should be observed at meter wavelengths by the next-generation radio telescopes such as the proposed Square Kilometer Array (SKA). Here we show that the extragalactic radio sources provide a serious contamination to the brightness temperature fluctuations expected in the redshifted 21 cm emission from the IGM at high redshifts. Unless the radio source population cuts off at flux levels above the planned sensitivity of SKA, its clustering noise component will dominate the angular fluctuations in the 21 cm signal. The integrated foreground signal is smooth in frequency space, and it should nonetheless be possible to identify the sharp spectral feature arising from the nonuniformities in the neutral hydrogen density during the epoch when the first UV sources reionize the IGM.