Euclid Q1 Exposes AGN Bias in Galaxy SED Fits, Forcing Recalibration of Cosmic Evolution Models Ahead of Stage-IV Surveys

The Euclid Quick Data Release (Q1) paper demonstrates that active galactic nuclei (AGN) emission systematically skews spectral energy distribution (SED) fits, inflating stellar masses by up to 0.3 dex and altering star-formation rates in affected galaxies. This preprint from the Euclid Collaboration analyzes early wide-field photometry, employing multi-component SED modeling that includes AGN templates versus standard galaxy-only fits on a sample of roughly 10^5 sources at 0.5 < z < 3. Methodology relies on Bayesian fitting codes applied to VIS and NISP bands supplemented by ancillary data, revealing that type-1 AGN hosts show the strongest biases while type-2 contributions remain subtler. Limitations include the preliminary calibration of Q1 photometry, incomplete spectroscopic validation, and restriction to relatively bright objects, making results indicative rather than definitive for the full survey. This finding connects directly to tensions in prior cosmology analyses from DES and KiDS, where unaccounted AGN contamination likely contributed to discrepancies in inferred halo occupation distributions. Related work in Salvato et al. (2022, A&A) on X-ray selected AGN similarly flagged SED biases but lacked Euclid's statistical power, while a 2023 MNRAS study by Ni et al. on mock catalogs predicted exactly these mass offsets yet underestimated their redshift evolution. The overlooked implication is that these systematics will propagate into weak-lensing and galaxy-clustering pipelines for Euclid, Roman, and LSST unless AGN fractions are jointly modeled with cosmological parameters, potentially shifting constraints on dark energy equation-of-state by several percent.