Testing Velocity Field Lensing on Realistic Galaxy Models
  Weak gravitational lensing shear could be measured far more precisely if
information about unlensed attributes of source galaxies were available. Disk
galaxy velocity fields supply such information, at least in principle, with
idealized models predicting orders of magnitude more Fisher information when
velocity field observations are used to complement images. To test the level at
which realistic features of disk galaxies (warps, bars, spiral arms, and other
substructure) inject noise or bias into such shear measurements, we fit an
idealized disk model, including shear, to unsheared galaxies in the Illustris
TNG simulation. The inferred shear thus indicates the extent to which unmodeled
galaxy features inject noise and bias. We find that $\gamma_+$, the component
of shear parallel to the galaxy's first principal axis, is highly biased and
noisy because disks display a range of intrinsic axis ratios from 0.8-1. The
other shear component, $\gamma_\times$, shows little bias and is well-described
by a double Gaussian distribution with central core scatter
$\sigma_{\text{core}} \approx$ 0.03, with low-amplitude, broad wings. This is
the first measurement of the natural noise floor in the proposed velocity field
lensing technique. We conclude that the technique will achieve impressive
precision gains for measurements of $\gamma_\times$, but little gain for
measurements of $\gamma_+$.
