2025ApJ...988..279Z

Gravitational lensing by massive galaxy clusters enables faint distant galaxies to be more abundantly detected than in blank fields, thereby allowing one to construct galaxy luminosity functions (LFs) to an unprecedented depth at high redshifts. Intriguingly, photometric redshift catalogs constructed from the Hubble Frontier Fields survey display an excess of z ≳ 4 galaxies in the cluster lensing fields that is not seen in accompanying blank parallel fields. The observed excess, while possibly a gift of gravitational lensing, could also be from misidentified low-z contaminants having similar spectral energy distributions as high-z galaxies. In the latter case, the contaminants may result in nonphysical upturns in UV LFs and/or washed-out faint-end turnovers predicted by contender cosmological models to Lambda cold dark matter (ΛCDM). Here, we employ the concept of magnification bias to perform the first statistical estimation of contamination levels in Hubble Frontier Fields lensing field photometric redshift catalogs. While we were able to reproduce a lower-z lensed sample, it was found ∼56% of the 3.5 < z_phot < 5.5 samples are likely low-z contaminants. Widely adopted Lyman-break-galaxy-like selection rules in the literature may give a “cleaner” sample magnification-bias-wise but we warn readers the resulting sample would also be less complete. Individual mitigation of contaminants is arguably the best way to investigate the faint high-z Universe, and this may be made possible with JWST observations.