Molecular Signals from Primordial Clouds at High Redshift

Abstract

The possibility of detecting cosmological signals from the postrecombination universe is one of the main aims of modern cosmology. In a previous paper we emphasized the role that elastic resonant scattering through LiH molecules can have in dumping primary cosmic background radiation (CBR) anisotropies and raising secondary signals. Here we extend our analysis to all the evolutionary stages of a primordial cloud, starting with the linear phase, through the turnaround, and to the nonlinear collapse. We have done calculations for protoclouds in a cold dark matter (CDM) scenario and, more generally, for a set of clouds with various masses and various turnaround redshifts, in this case without referring to any particular structure formation scenario. We found that the first phase of collapse, for t/t<SUB>free-fall</SUB> = 0.05-0.2 is the best one for simultaneous detection of the first two LiH rotational lines. The observational frequency falls between 30 and 250 GHz, and the line width Δν/ν is between 10<SUP>-5</SUP> and 10<SUP>-4</SUP>. As far as we know, this is the most favorable process to detect primordial clouds before they begin star formation processes.