NEUTRAL HYDROGEN AT HIGH REDSHIFTS AS A PROBE OF STRUCTURE FORMATION .2. LINE-PROFILE OF A PROTOCLUSTER

The formation of structures at z less than or similar to 10.0 can be p robed using the 21-cm line emission from the neutral hydrogen. Two of us (KS and TP, Paper I) previously computed the expected abundance of protoclusters as a function of the flux density at various redshifts, in the cold dark matter (CDM) and the hot dark matter (HDM) models. As a complement to Paper I, here we work out in detail how the HI line p rofile from a spherically symmetric protocluster evolves as it decoupl es from Hubble expansion and collapses. Our paradigm for structure for mation is the CDM model where structures form hierarchically. Neutral hydrogen, in the small-scale clumps that form the protocluster, is the source of HI line emission. We find that the peak fluxes of the HI li ne profile in this model are typically of order 0.5-0.7 mJy, while the widths (FWHM) are of order 0.3-1.8 MHz. The major uncertainty in our calculations is the fraction of mass of the protocluster in the form o f neutral hydrogen. If the neutral hydrogen fraction f is of the order of the value we have adopted (f = 0.025) in our calculations or great er, then a typical protocluster could indeed be detectable by future f acilities, like the Giant Metrewave Radio Telescope (GMRT) which is cu rrently being built in India. If the neutral hydrogen fraction is much less than the value we have adopted, then a more sensitive instrument is needed to detect the HI line emission from a typical protocluster.