SN 2023adsy: A normal type Ia Supernova at

Context. Supernovae (SNe) discovered in high-redshift (z > 2) galaxies by the James Webb Space Telescope (JWST) provide a unique opportunity to extend the Hubble diagram beyond z ∼ 1.5 and constrain the cosmic star formation rate in the early Universe. Aims. SN 2023adsy, a type Ia supernova discovered by JWST at z = 2.9, was found to be a peculiar event, since it is extremely red and faint, but it shows a very similar rest-frame light-curve decline rate to the majority of low-redshift SNe Ia. We examine whether the red color and faint peak magnitude could also be explained by significant reddening and extinction due to dust within the host galaxy. Methods. We used two light-curve fitting codes, SALT3-NIR and BayeSN, which have templates extended to the near-infrared, to re-fit the published NIRCam photometry, assuming a “normal” SN Ia and Milky-Way dust extinction law. We also examined NIRCam photometry of the host galaxy taken before the SN discovery by comparing its spectral energy distribution with galaxy templates. Results. The NIRCam photometry can be fit reasonably well with a slowly declining but otherwise “normal” SN Ia template that suffers significant reddening (E(B − V)host ≳ 0.5 mag). Photometry of the host galaxy suggests a blue, star-forming galaxy, where the presence of a significant amount of dust cannot be ruled out. Conclusions. A comparison of the inferred luminosity distance with the prediction of the ΛCDM cosmology on the Hubble diagram suggests no significant evolution of the SN Ia peak luminosity at z > 2 redshifts. We also show that the discovery of a single SN Ia between 2 < z < 3 within the area of the JADES survey during approximately one year is consistent with the current estimates for the SN Ia rates at such redshifts.