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

TL;DR

This paper reports JWST observations of a normal Type Ia supernova at redshift 2, providing key insights into its properties and supporting the robustness of SN Ia standardization for high-redshift cosmology.

Contribution

First spectroscopic confirmation of a normal SN Ia at z=2 with detailed analysis supporting its use as a cosmological standard candle.

Findings

SN 2025ogs exhibits standard SN Ia light curve and spectral features at z=2.

Luminosity distance aligns with flat ΛCDM and current cosmological constraints.

Absorption features show lower blueshift compared to low-z SNe Ia, indicating potential evolutionary effects.

Abstract

The Nancy Grace Roman Space Telescope will provide a revolutionary measurement of the Universe's expansion kinematics, driven by dark matter and dark energy, out to $z \approx 3$. 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 ($z \gtrsim 1.5$), 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 $z=2.05\pm 0.01$. This SN offers a key point of comparison for interpreting future high-redshift SN Ia samples. The NIRCam light curve indicates a blue color ($B - V = -0.27 \pm 0.06$ mag) and a moderately fast decline ($\Delta m_{15}(B) = 1.55 \pm 0.15$ mag), both within standard criteria for inclusion in cosmological analyses. Its luminosity distance is in $1.0\sigma$ agreement with a standard flat $\Lambda$CDM model, as well as with current cosmological constraints from the Dark Energy Survey (DES 5yr) 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-ultraviolet 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&K, O I $\lambda7774$, and the Ca II NIR 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 $z \approx 2$, and also highlight the importance of JWST spectroscopy for uncovering evolutionary effects that could impact Roman's high-precision cosmology.