A Direct DESI–SDSS Two-Fibre Test of Aperture Bias in Optical Emission-Line Diagnostics

Fixed angular apertures sample different physical regions of nearby galaxies and can therefore bias optical emission-line diagnostics. We use 20,545 high-confidence emission-line galaxies observed by both the Dark Energy Spectroscopic Instrument (DESI) and the Sloan Digital Sky Survey (SDSS) as a two-fibre experiment in which the DESI 1.5 arcsec fibre is nested within the SDSS 3 arcsec fibre. To isolate aperture effects from line-measurement systematics, we refit every spectrum using the same eMILES stellar-continuum and emission-line model while preserving the wavelength-dependent line-spread function of each spectrum. This common refit yields 20,200 quality-controlled matched galaxies. Relative to SDSS, the smaller DESI aperture produces small but highly significant offsets: Δlog(N2)=+0.0067±0.0002, Δlog(S2)=−0.0148±0.0003, Δlog(O3)=−0.0246±0.0006, ΔO3N2=−0.0327±0.0007, and Δlog(Hα/Hβ)=−0.0195±0.0003, where Δ denotes DESI minus SDSS. These offsets persist in Baldwin–Phillips–Terlevich-selected star-forming galaxies and in a near-concentric subset. Their amplitudes vary across the redshift distribution and retain a measurable dependence on apparent fibre coverage after redshift is controlled, showing that redshift and relative aperture coverage jointly modulate the aperture terms. We conclude that, for high-S/N emission-line galaxies, DESI–SDSS comparisons carry measurable, diagnostic-dependent fibre-aperture terms, even when continuum modelling, emission-line fitting, and spectral-resolution treatment are held fixed.