Professor T. Padmanabhan
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Item Zeldovich approximation and the probability distribution for the smoothed density field in the nonlinear regime(American Astronomical Society, 1993-06-20) Padmanabhan, T.; Subramanian, KandaswamyThe study of large-scale structure in the Universe is often based on the observed density distribution of matter smoothed by a suitable filter function. The probability distribution for this smoothed density field in the nonlinear regime is studied using the Zel'dovich approximation. When the shear term of the velocity field is not too large, one can obtain a reasonably good analytic approximation tho this probability distribution. The properties of this distribution are discussed and compared with other attempts along similar lines.Item Galaxy Formation(Astronomical Society of India, 1992-10-15) Padmanabhan, T.; Subramanian, KandaswamyItem Focusing equations, caustics and the condition for multiple imaging by thick gravitational lenses(Royal Astronomical Society, 1987-12-27) Padmanabhan, T.; Subramanian, KandaswamyThe condition for the production of multiple images by an arbitrary (thick or thin) gravitational lens are studied. We show that the necessary and sufficient condition for the production of multiple images by a lens is the following: The lens should produce a point conjugate to the observer, along some null geodesic, at an affine distance smaller than that of the source. It is shown that previous results on multiple imaging by thin lenses can be obtained as a special case. We also show that a thick lens cannot be more efficient than a suitably designed thin lens for the production of multiple images.Item Aspects of Zeldovich approximation(American Astronomical Society, 1993-04-15) Padmanabhan, T.; Subramanian, KandaswamyA generalized version of the Zel'dovich approximation which is applicable in both the radiation-dominated and the matter dominated epochs is presented. This approximation allows one to follow the growth of inhomogeneities from the time the mode enters the hubble radius until it turns around. Comparison of the results with the standard spherical model shows that the analytical approximation is quite good even in the nonlinear regime. Detailed application to cold dark matter models and seeded models are given.Item 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, KandaswamyThe 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.Item 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.Item Neutral hydrogen at high redshifts as a probe of structure formation(Indian Academy of Sciences, 1995-06-10) Kumar, A.; Subramanian, Kandaswamy; Padmanabhan, T.Structure formation at < 10.0 can be detected and studied using the 21 cm line emission from the neutral hydrogen. Two of us (Suramanian & Padmanabhan 1993, Paper I) had earlier computed the expected abundance of Procrustes as a function of the flux density a function of the flux density at various redshifts, in the detail how the HI line profile from a single spherically symmetric protocluster evolves as it decouples from hubble expansion and colapses. We find peak fluxes of the HI line profile to be typically of order 0.4-0.8 mJy while the widths (FWHM) are of order 0.6-1.5 Mhz. Such protoclusters could be detectable by the Giant Metrewave Radio Telescope (GMRT) which is being built in India.