Professor T. Padmanabhan

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Author: search.filters.author.Subramanian, KandaswamySubject: search.filters.subject.Dark matter

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    Neutral hydrogen at high redshifts as a probe of structure formation- II. Line profile of a protocluster
    (Wiley-Blackwell, 1994-09-05) Kumar, A.; Padmanabhan, T.; Subramanian, Kandaswamy
    The formation of structures at z ≤ 10.0 can be probed using the 21-cm line emisssion 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 H1 line profile from a spherically symmetric protocluster evolves as it decouples from Hubble expansion structures form hierarchically. Neutral hydrogen, in the small-scale clumps that from the protocluster, is the source of H1line 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 of 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 greater, then a typical protocluster could indeed be detectable by future facilities, like the Giant Metrewave Radio Telescope (GMRT) which is currently 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 H1 line emission from a typical protocluster.
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    Neutral hydrogen at high redshifts as a probe of structure formation: 1. Post-COBE analysis of CDM and HDM models
    (Royal Astronomical Society, 1993-05-11) Subramanian, Kandaswamy; Padmanabhan, T.
    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 11DM 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≃(10-8-10-7)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 11DM models - when normalized using COBE results - do not lead to a detectable number of sources (`pancakes') at redshifts z≳2.