Magnification Bias of Distant Galaxies in the Hubble Frontier Fields:
  Testing Wave vs. Particle Dark Matter Predictions
  Acting as powerful gravitational lenses, the strong lensing galaxy clusters
of the deep Hubble Frontier Fields (HFF) program permit access to
lower-luminosity galaxies lying at higher redshifts than hitherto possible. We
analyzed the HFF to measure the volume density of Lyman-break galaxies at $z >
4.75$ by identifying a complete and reliable sample up to $z \simeq 10$. A
marked deficit of such galaxies was uncovered in the highly magnified regions
of the clusters relative to their outskirts, implying that the magnification of
the sky area dominates over additional faint galaxies magnified above the flux
limit. This negative magnification bias is consistent with a slow rollover at
the faint end of the UV luminosity function, and indicates a preference for
Bose-Einstein condensate dark matter with a light boson mass of $m_\mathrm{B}
\simeq 10^{-22} \, \mathrm{eV}$ over standard cold dark matter. We emphasize
that measuring the magnification bias requires no correction for multiply
lensed images (with typically three or more images per source), whereas
directly reconstructing the luminosity function will lead to an overestimate
unless such images can be exhaustively matched up, especially at the faint end
that is accessible only in the strongly lensed regions. In addition, we
detected a distinctive downward transition in galaxy number density at $z
\gtrsim 8$, which may be linked to the relatively late reionization reported by
Planck. Our results suggests that JWST will likely peer into an "abyss" with
essentially no galaxies detected in deep NIR imaging at $z > 10$.
