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

TL;DR

This study uses JWST NIRCam photometry and NIRSpec spectroscopy to characterize SN 2025ogs, a spectroscopically normal Type Ia supernova at $z = 2.05 \pm 0.01$, as a benchmark for high-redshift SN Ia evolution. The NIRCam light curve shows a blue color of $B - V = -0.27 \pm 0.06$ mag and a decline rate $\Delta m_{15}(B) = 1.55 \pm 0.15$ mag, with a luminosity distance compatible with a flat $\Lambda$CDM model and with DES 5yr and Pantheon+ constraints. The NIRSpec spectrum reveals hallmark SN Ia absorption features at peak, with rest-frame optical color, near-UV properties, and Si II strengths consistent with the inferred decline rate, although several features exhibit lower blueshifts than low-redshift SNe Ia. Together, these results indicate that SN Ia standardization remains robust at $z \approx 2$, while illustrating how JWST spectroscopy can reveal evolutionary effects that might impact Roman's high-precision cosmology.

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 ($Δm_{15}(B) = 1.55 \pm 0.15$ mag), both within standard criteria for inclusion in cosmological analyses. Its luminosity distance is in $1.0σ$ agreement with a standard flat $Λ$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.

Figures (2)

• Figure 1: False color images of SN 2025ogs and its host galaxy using F150W (Blue), F200W (Green), and F356W (Red). The left panel is the template image obtained from the PRIMER program (PID: 1837, dunlop_primer_2021) and the right panel is our discovery image from COSMOS-3D (PID: 5893). The SN is well-separated from its host galaxy at a projected distance of 8.1 kpc.
• Figure 2: Comparison of our NIRSpec prism spectrum of SN 2025ogs (black) to SN 2011fe (blue) at 3.4 days after peak brightness. The spectrum of SN 2011fe has been convolved with the prism dispersion function, rebinned, and scaled to match the observation.