2025ApJ...995L...5Z

We present observational evidence that intense ionizing radiation from a luminous quasar suppresses nebular emission in nearby galaxies on intergalactic scales at z = 6.3. Using JWST/NIRCam grism spectroscopy from the Slitless Areal Pure-Parallel High-Redshift Emission survey and Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization programs, we identify a moderate but statistically significant decline in [O III] λ5008 luminosity relative to the UV continuum (L₅₀₀₈/L₁₅₀₀) among galaxies within ∼7 comoving Mpc (cMpc) of the quasar J0100+2802, the most UV-luminous quasar known at this epoch (M₁₄₅₀ = -29.26). While L₁₅₀₀ remains roughly constant with transverse distance, L₅₀₀₈ increases significantly, suggesting suppression of very recent star formation toward the quasar. The effect persists after controlling for completeness, local density, and UV luminosity, and correlates with the projected photoionization-rate profile Γ_qso. A weaker but directionally consistent suppression in L₅₀₀₈/L₁₅₀₀ is also observed along the line of sight. The transverse suppression radius (∼7 cMpc) implies a recent radiative episode with a cumulative duration ∼3.1 Myr, shorter than required for thermal photoheating to dominate and thus more naturally explained by rapid H₂ photodissociation and related radiative processes. Environmental effects alone appear insufficient to explain the signal. Our results provide direct, geometry-based constraints on large-scale quasar radiative feedback and recent quasar lifetimes.