Galaxy Ellipticity Measurements in the Near-Infrared for Weak Lensing
  We investigate the value of the near-infrared imaging from upcoming surveys
for constraining the ellipticities of galaxies. We select galaxies between 0.5
< z < 3 that are brighter than expected Euclid sensitivity limits from the
GOODS-S and N fields in CANDELS. The co-added CANDELS/HST V+I and J+H images
are degraded in resolution and sensitivity to simulate Euclid-quality optical
and near-infrared (NIR) images. We then run GALFIT on these simulated images
and find that optical and NIR provide similar performance in measuring galaxy
ellipticities at redshifts 0.5 < z < 3. At z > 1.0, the NIR-selected source
density is higher by a factor of 1.4 and therefore the standard error in
NIR-derived ellipticities is about 30% smaller, implying a more precise
ellipticity measurement. The good performance of the NIR is mainly because
galaxies have an intrinsically smoother light distribution in the NIR bands
than in the optical, the latter tracing the clumpy star-forming regions. In
addition, the NIR bands have a higher surface brightness per pixel than the
optical images, while being less affected by dust attenuation. Despite the
worse spatial sampling and resolution of Euclid NIR compared to optical, the
NIR approach yields equivalent or more precise galaxy ellipticity measurements.
If systematics that affect shape such as dithering strategy and point spread
function undersampling can be mitigated, inclusion of the NIR can improve
galaxy ellipticity measurements over all redshifts. This is particularly
important for upcoming weak lensing surveys, such as with Euclid and WFIRST.
