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Precision Weak Gravitational Lensing Using Velocity Fields: Fisher Matrix Analysis


Weak gravitational lensing measurements based on photometry are limited by shape noise: the variance in the unknown unlensed orientations of the source galaxies. If the source is a disk galaxy with a well-ordered velocity field, however, velocity field data can support simultaneous inference of the shear, inclination, and position angle, virtually eliminating shape noise. We use the Fisher information matrix formalism to forecast the precision of this method in the idealized case of a perfectly ordered velocity field defined on an infinitesimally thin disk. For nearly face-on targets, one shear component, γ×, can be constrained to 0.003 9025/Ionpix, where I0 is the signal-to-noise ratio of the central intensity pixel and npix is the number of pixels across a diameter enclosing 80% of the light. This precision degrades with inclination angle by a factor of 3 by i = 50°. The uncertainty on the other shear component, γ+, is about 1.5 (7) times larger than the γ× uncertainty for targets at i = 10° (50°). For an arbitrary galaxy position angle on the sky, these forecasts apply not to γ+ and γ× as defined on the sky but to two eigenvectors in (γ+, γ×, μ) space, where μ is the magnification. We also forecast the potential of less expensive partial observations of the velocity field, such as slit spectroscopy. We conclude by outlining some ways in which real galaxies depart from our idealized model and thus create random or systematic uncertainties not captured here. In particular, our forecast γ× precision is currently limited only by the data quality rather than scatter in galaxy properties because the relevant type of scatter has yet to be measured.

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