Neutral hydrogen at high redshifts as a probe of structure formation - I. Post-COBE analysis of CDM and HDM models

The structures that form in the Universe at redshifts z ≲ 10 can be detected and studied using the redshifted 21-cm line emission from neutral hydrogen. We compute the expected comoving number density, N, of protocondensates that will emit a flux higher than S, at various redshifts, in the CDM and HDM models. The models are normalized using COBE results. Our results are compared with the present and expected future sensitivities of various telescopes for the detection of protocondensates-. In the CDM models the predicted maximum fluxes at a redshift z ≃ 3.3 are about (1.5–3) mJy and $$N \simeq ({10}^{-8}-{10}^{-7})\, \text {Mpc}^{-3}$$ . These protocondensates cannot be detected with present sensitivities, but will become detectable in the near future with improved sensitivities. At lower redshifts, the detectability of these structures critically depends on their neutral hydrogen content. In the redshift range around z≃ 5, individual protocondensates will not be detectable. The excess variance due to fluctuations with small density contrasts will, however, be detectable with somewhat large (say, about 60-h) integration time. At still higher redshifts, it will be virtually impossible to see any signal, even with such a large integration time. Biased CDM models predict larger fluxes, but somewhat lower abundances. Finally, the HDM models – when normalized using COBE results – do not lead to a detectable number of sources (‘pancakes’) at redshifts z ≳ 2.