Detecting Gravitational Lensing Cosmic Shear from Samples of Several Galaxies Using Two-dimensional Spectral Imaging

Studies of weak gravitational lensing by large-scale structures require the measurement of the distortions introduced to the shapes of distant galaxies at the few percent level by anisotropic light deflection along the line of sight. In order to detect this signal on 1'-10' scales in a particular field, accurate measurements of correlations between the shapes of order 103-104 galaxies are required. This large-scale averaging is required to accommodate the unknown intrinsic shapes of the background galaxies, even with careful removal of systematic effects. Here an alternative is discussed. If it is possible to measure accurately the detailed dynamical structure of the background galaxies, in particular rotating disks, then it should be possible to measure directly the cosmic shear distortion since it generally leads to a non-self-consistent rotation curve. Narrow spectral lines and an excellent two-dimensional spatial resolution are required. The ideal lines are CO rotational transitions, and the ideal telescope is the Atacama Large Millimeter Array.