HST/WFC3 obs. of z~2-8 galaxies in 4 HFF clusters

Remarkably, however, no correlation is found between the surface density of faint galaxies and the predicted shear, using 87 high-magnification ({mu}=10-100) z~2-8 galaxies seen behind the first four HFF clusters. This can only be the case if faint (~-15mag) galaxies have significantly smaller sizes than more luminous galaxies, i.e., <~30mas or 160-240pc. As a second size probe, we rotate and stack 26 faint high-magnification sources along the major shear axis.

Less elongation is found even for objects with an intrinsic half-light radius of 10mas. Together, these results indicate that extremely faint z~2-8 galaxies have near point-source profiles (half-light radii <30mas and perhaps 5-10mas). These results suggest smaller completeness corrections and hence shallower faint-end slopes for the z~2-8 LFs than derived in some recent studies (by {Delta}{alpha}>~0.1-0.3).