Probing the bispectrum at high redshifts using 21 cm HI observations

Abstract

Observations of fluctuations in the redshifted 21 cm radiation from neautral hydrogen (HI) are perceived to be an important future probe of the universe at high redshifts. Under the assumption that at redshifts z less than 6 (Post-Reionization Era), the HI 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 GMRT, 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 HI signal is found to be uncorrelated beyond frequency separations of 1.3 MHz whereas the continuum sources of continuum are expected to be correlated across much larger frequencies. This signature can in principle be used to distinguish the HI 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 HI at high redshifts, this having possible implications for theories of galaxy formation.