weak lensing measurements?
Priyamvada Natarajan , Robert G. Crittenden,
Ue-Li Pen \& Tom Theuns
, PASA, 18 (2), in press.
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Introduction
Gravitational lensing can be used to map the detailed distribution of matter in the Universe over a range of scales (Gunn 1967). Systematic distortions in the shapes and orientations of high redshift background galaxies (weak lensing) induced by mass inhomogeneities along the line of sight can be measured statistically (Gunn 1967; Blandford et al. 1991; Miralda-Escude 1991; Kaiser 1992; see a recent review by Bartelmann & Schneider 1999). The lensing effect depends only on the projected surface mass density and is independent of the luminosity or the dynamical state of the mass distribution. Thus, this technique can potentially provide invaluable constraints on the distribution of matter in the Universe and the underlying cosmological model (Bernardeau, van Waerbeke & Mellier 1997). There has been considerable progress in theoretical calculations of the effects of weak lensing by large-scale structure, both analytically and using ray-tracing through cosmological N-body simulations (Kaiser 1992; Bernardeau, van Waerbeke & Mellier 1997; Jain & Seljak 1997; Jain, Seljak & White 2000). Recently, several teams have also reported observational detections of `cosmic shear' - weak lensing on scales ranging from an arc-minute to ten arc-minutes (see Fig. 1; Van Waerbeke et al. 2000; Bacon, Refregier & Ellis 2000; Wittman et al. 2000; Kaiser, Wilson & Luppino 2000). At present, these studies are limited by observational effects, such as shot noise due to the finite number of galaxies and the accuracy with which shapes can actually be measured given the optics and seeing (Kaiser 1995; Bartelmann & Schneider 1999). In addition, the intrinsic ellipticity distribution of galaxies and their redshift distribution is still somewhat uncertain. These observational difficulties can be potentially overcome with more data. However, an important theoretical issue remains. In modeling the distortion produced by lensing, it is assumed that the a priori intrinsic correlations in the shapes and orientations of background galaxies are negligible. Correlations in the intrinsic ellipticities of neighboring galaxies are expected to arise from the galaxy formation process, for example as a consequence of correlations between the angular momenta of galaxies when they assemble. The strength of these correlations can be computed in linear theory, in the context of Gaussian initial fluctuations.Next Section: Schematic Outline
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