2000 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/629
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Item Using Gravitational Lensing to study damped Lyman - alpha clouds(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-α clouds) at high redshift by carrying out deep radio observations in the fields of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magnifies the flux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We find that observations at a detection threshold of 50 Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the field of a cluster, provided the clouds have HI masses in the range 5 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster field becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a definitive statement is difficult owing to uncertainties in the HI content at the redshifts corresponding to these fre- quencies. Observations at a detection threshold of 2 Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the field of every cluster provided the clouds have HI masses in the range 2 × 107 M⊙ ≤ MHI ≤ 109 M⊙. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.Item Observing high-redshift Supernovae in lensed galaxies(2000-01-28) Saini, Tarun Deep; Raychaudhury, Somak; Shchekinov, Yuri A.Supernovae in distant galaxies that are grav- tationally lensed by foreground galaxy clusters make ex- cellent cosmological candles for measuring quantities like the density of the Universe in its various components and the Hubble constant. Distant supernovae will be more eas- ly detectable since foreground cluster lenses would mag- nify such supernovae by up to 3–4 magnitudes. We show that in the case of the lens cluster Abell 2218, the de- tectability of high-redshift supernovae is significantly en- hanced due to the lensing effects of the cluster. Since ensed supernovae will remain point images even when their host galaxies are stretched into arcs, the signal-to- noise ratio for their observation will be further enhanced, typically by an order of magnitude. We recommend moni- toring well-modelled clusters with several known arclets for the detection of cosmologically useful SNe around z = 1 and beyond.Item Lens mapping algorithm for weak lensing(2000-09-24) Saini, Tarun Deep; Raychaudhury, SomakWe develop an algorithm for the reconstruction of the two-dimensional mass distribution of a gravitational lens from the observable distortion of background galaxies. From the measured reduced shear γi/(1 − κ) the lens mapping is obtained, from which a mass distribution is derived. This is unlike other methods where the convergence κ is directly obtained. We show that this method works best for sub-critical lenses, but can be applied to a critical lens away from the critical lines. For finite fields the usual mass-sheet degeneracy is shown to exist in this method as well. We show that the algorithm reproduces the mass distribution within acceptable limits when applied to simulated noisy data.