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

Abstract

ABSTRACT Observations of fluctuations in the redshifted 21 cm radiation from neutral hydrogen(HI) 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 HI tracesthe underlying dark matter with a possible bias, we investigate the possibility of usingobservations of redshifted 21 cm radiation to detect the bispectrum arising from non-linear gravitational clustering and from non-linear bias. We find that the expectedsignal is ∼ 0.1mJy at 325MHz (z = 3.4) for the small baselines at the GMRT, thestrength being a few times largerat higher frequencies (610MHz,z = 1.3). Further, themagnitude ofthe signalfromthe bispectrum is predictedto be comparabletothat fromthe 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 ∼ 1.3MHzwhereas the continuum sources of continuum are expected to be correlated acrossmuch larger frequencies. This signature can in principle be used to distinguish the HIsignal from the contamination. We also consider the possibility of using observationsof the bispectrum to determine the linear and quadratic bias parameters of the HI athigh redshifts, this having possible implications for theories of galaxy formation.Key words: cosmology: large scale structure of universe - intergalactic medium -diffuse radiation