Probing the bispectrum at high redshifts using 21-cm H I observations

Observations of fluctuations in the redshifted 21-cm radiation from neutral hydrogen (H i) are perceived to be an important future probe of the universe at high redshifts. Under the assumption that at redshifts z≤ 6 (post-reionization era) the H i traces the underlying dark matter with a possible bias, we investigate the possibility of using observations of redshifted 21-cm radiation to detect the bispectrum arising from non-linear gravitational clustering and from non-linear bias. We find that the expected signal is ∼ 0.1 mJy at 325 MHz (z= 3.4) for the small baselines at the Giant Metrewave Radio Telescope, the strength being a few times larger at higher frequencies (610 MHz, z= 1.3). Further, the magnitude of the signal from the bispectrum is predicted to be comparable to that from the power spectrum, allowing a detection of both in roughly the same integration time. The H i signal is found to be uncorrelated beyond frequency separations of ∼1.3 MHz whereas the continuum sources of contamination are expected to be correlated across much larger frequencies. This signature can in principle be used to distinguish the H i signal from the contamination. We also consider the possibility of using observations of the bispectrum to determine the linear and quadratic bias parameters of the H i at high redshifts, this having possible implications for theories of galaxy formation.