2025ApJ...981...19L

JWST is revealing a remarkable new population of high-redshift (z ≳ 4), low-luminosity active galactic nuclei in deep surveys and detecting the host galaxy's stellar light in the most luminous and massive quasars at z ∼ 6 for the first time. Recent findings claim that supermassive black holes (SMBHs) in these systems are significantly more massive than predicted by the local black hole (BH) mass-stellar mass (${{ \mathcal M }}_{\mathrm{BH}}-{{ \mathcal M }}_{\star }$) relation and that this is not due to sample selection effects. Through detailed statistical modeling, we demonstrate that the coupled effects of selection biases (i.e., finite detection limit and requirements for detecting broad lines) and measurement uncertainties can largely explain the reported offset and flattening in the observed ${{ \mathcal M }}_{\mathrm{BH}}-{{ \mathcal M }}_{\star }$ relation toward the upper envelope of the local relation, even for those at ${{ \mathcal M }}_{\mathrm{BH}}\,\lt 1{0}^{8}\,{M}_{\odot }$. We further investigate the possible evolution of the ${{ \mathcal M }}_{\mathrm{BH}}-{{ \mathcal M }}_{\star }$ relation at z ≳ 4 with careful treatment of observational biases and consideration of the degeneracy between intrinsic evolution and dispersion in this relation. The bias-corrected intrinsic ${{ \mathcal M }}_{\mathrm{BH}}-{{ \mathcal M }}_{\star }$ relation in the low-mass regime (${{ \mathcal M }}_{\star }\lesssim 1{0}^{10}\,{M}_{\odot }$) suggests a large population of low-mass BHs (${{ \mathcal M }}_{\mathrm{BH}}\lesssim 1{0}^{5}\,{M}_{\odot }$), possibly originating from lighter seeds, may remain undetected or unidentified. These results underscore the importance of forward modeling observational biases to better understand BH seeding and SMBH-galaxy coevolution mechanisms in the early universe, even with the deepest JWST surveys.