One of the primary science goals of Euclid is to constrain cosmological parameters through weak gravitational lensing. For the weak lensing shape measurement, the Euclid survey will detect galaxies in a broad optical R+i+z band (VIS), which reaches a 10 sigma depth of 24.5 AB magnitude. In this study, we demonstrate that co-added J+H-band near-infrared (NIR) images at Euclid's resolution and depth can yield better ellipticities of galaxies at redshifts 0.5<z<3 than such optical images. There are several reasons for this better performance: Due to the morphological k-correction, galaxies have an intrinsically smoother shape distribution in the NIR bands than in the VIS, the latter mostly tracing the rest-frame UV light which follows the clumpy distribution of star-forming regions. Also, near-IR (J+H) images of galaxies have a higher surface brightness with more than three times the number of source photons per pixel and are less affected by dust extinction. Another benefit of using the near-IR bands is that they are sensitive to more than a factor of 2 more galaxies at z>1.5. We select galaxies at 0.5<z<3 satisfying the Euclid sensitivity limit from CANDELS GOODS-S and GOODS-N fields and use F606w(V), F814w(I), F125w(J) and F160w(H) HST images of them. We then simulate Euclid VIS and near-IR images by degrading the co-added CANDELS/HST V+I and J+H images to the Euclid resolution, respectively, and compare the ellipticities estimated from CANDELS (I or H) and Euclid (V+I or J+H) images. By doing so, we find 20% better ellipticity measurement in J+H than in VIS. Despite the worse spatial sampling of the Euclid imaging data in the NIR compared to VIS (0.3" vs. 0.1"), we can extract a robust ellipticity measurement from the NIR bands. Our study motivates the careful assessment of NIR shape systematics for future weak gravitational lensing surveys, such as with Euclid and WFIRST.
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018