The Extremely Metal-poor SN 2023ufx: A Local Analog to High-redshift Type II Supernovae

We present extensive observations of the Type II supernova (SN II) SN 2023ufx, which is likely the most metal-poor SN II observed to date. It exploded in the outskirts of a low-metallicity ( Z host ∼ 0.1 Z ⊙ ) dwarf ( M g = −13.39 ± 0.16 mag, r proj ∼ 1 kpc) galaxy. The explosion is luminous, peaking at M g ≈ −18.5 mag, and shows rapid evolution. The r -band (pseudobolometric) light curve has a shock-cooling phase lasting 20 (17) days followed by a 19 (23) day plateau. The entire optically thick phase lasts only ≈55 days following explosion, indicating that the red supergiant progenitor had a thinned H envelope prior to explosion. The early spectra obtained during the shock-cooling phase show no evidence for narrow emission features and limit the preexplosion mass-loss rate to M ̇ ≲ 10 − 3 M ⊙ yr −1 . The photospheric-phase spectra are devoid of prominent metal absorption features, indicating a progenitor metallicity of ≲0.1 Z ⊙ . The seminebular (∼60–130 days) spectra reveal weak Fe ii , but other metal species typically observed at these phases (Ti ii , Sc ii , and Ba ii ) are conspicuously absent. The late-phase optical and near-infrared spectra also reveal broad (≈10 4 km s −1 ) double-peaked H α , P β , and P γ emission profiles suggestive of a fast outflow launched during the explosion. Outflows are typically attributed to rapidly rotating progenitors, which also prefer metal-poor environments. This is only the second SN II with ≲0.1 Z ⊙ and both exhibit peculiar evolution, suggesting a sizable fraction of metal-poor SNe II have distinct properties compared to nearby metal-enriched SNe II. These observations lay the groundwork for modeling the metal-poor SNe II expected in the early Universe.