SN 2025ogs: A Spectroscopically Normal Type Ia Supernova at z = 2 as a Benchmark for Redshift Evolution

Abstract The Nancy Grace Roman Space Telescope (Roman) will provide a revolutionary measurement of the Universe’s expansion kinematics, driven by dark matter and dark energy, out to z ≈ 3. The accuracy of this measurement is predicated on the assumption that standardized Type Ia supernova (SN Ia) luminosities do not evolve with redshift. If present, SN Ia luminosity evolution is expected to be most detectable in the dark-matter-dominated era of the Universe ( z ≳ 1.5), with its effects becoming more easily distinguishable from dark energy variation at increasing redshift. We present JWST NIRCam and NIRSpec observations of SN 2025ogs, a normal SN Ia at z = 2.05 ± 0.01. This supernova offers a key point of comparison for interpreting future high-redshift SN Ia samples. The NIRCam light curve indicates a blue color ( B − V = −0.27 ± 0.06 mag) and a moderately fast decline (Δ m 15 ( B ) = 1.55 ± 0.15 mag), both within standard criteria for inclusion in cosmological analyses. Its luminosity distance is in 1.0 σ agreement with the standard flat ΛCDM model, as well as with current cosmological constraints from the Dark Energy Survey (5 yr) and Pantheon+. The NIRSpec spectrum displays all of the hallmark absorption features of a normal SN Ia observed at peak brightness. We find that the rest-frame optical color, rest-frame near-UV properties, and Si ii line strengths are all consistent with the moderately fast decline inferred from the light curve. Multiple absorption features (Ca ii H and K, O i λ 7774, and the Ca ii near-infrared triplet) all appear at a lower blueshift relative to a sample of low- z SNe Ia. Together, these results suggest that SN Ia standardization remains robust at z ≈ 2, and also highlight the importance of JWST spectroscopy for uncovering evolutionary effects that could impact Roman’s high-precision cosmology.