Nebular Spectra of the Extremely Metal-Poor SN II 2023ufx Over a Year After Explosion: A Massive Progenitor and Unique Circumstellar Environment

We present new observations of the metal-poor (< 0.1 Z) Type II supernova 2023ufx and its host galaxy. The deep nebular spectrum, obtained roughly a year after explosion, has a triple-peaked [O I] emission profile suggesting an asymmetric explosion. Comparison to nebular spectra models suggests a zero-age main sequence mass of M ZAMS about 25-35 M, which is supported by the low [Ca II]/[O I] emission-line ratio of about0.4. The diminishing, broad, boxy H α emission and flattening of the light curve in the late (about2 yr) photometry suggest a complex mass-loss history in the centuries to millennia before explosion. New optical and near-infrared imaging of the host galaxy confirms that it is a dwarf, with a stellar mass of 10 6.6 plus or minus0.1 M and a SFR of 10-2.5 plus or minus0.1 M /yr. Results from both SED fitting and galaxy stellar mass-metallicity scaling relations all lead to an environmental metallicity estimate of 0.02-0.07 Z. Taken together, these observations confirm that SN 2023ufx is the explosion of a very metal-poor, heavily stripped red supergiant.