JWST NIRSpec Spectroscopy of the Remarkable Bright Galaxy GHZ2/GLASS-z12 at Redshift 12.34

We spectroscopically confirm the M_ UV = -20.5 mag galaxy GHZ2/GLASS-z12 to be at redshift z=12.34. The source was selected via NIRCam photometry in GLASS-JWST Early Release Science data, providing the first evidence of a surprising abundance of bright galaxies at z ≳ 10. The NIRSpec PRISM spectrum is remarkable and unlike any local analog. It shows significant detections of N IV, C IV, He II, O III, C III, O II, and Ne III lines, and the first detection in a high-redshift object of the O III Bowen fluorescence line at 3133 Å rest-frame. The prominent C IV line with rest-frame equivalent width (EW) ∼ 46 Å puts GHZ2 in the category of extreme C IV emitters characterised by hard radiation fields. GHZ2 displays UV lines with EWs that are only found in active galactic nuclei (AGNs) or composite objects at low/intermediate redshifts, and UV line-intensity ratios that are compatible both with AGNs and star formation in a low-metallicity environment. The nondetection of the very high-ionization lines [Ne IV] and [Ne V], and the remarkable similarity between GHZ2 and other known C IV emitters, favors a scenario in which the high ionizing output is due to very low metallicity, massive stars forming in a dense environment. We estimate a metallicity ≲ 0.1 Z/ Z_⊙, a high ionization parameter logU > -2, a N/O abundance 4–5 times the solar value, and a subsolar C/O ratio similar to the recently discovered class of nitrogen-enhanced objects at high redshift. Considering its abundance patterns and the high stellar mass density (10^4 M_⊙ pc^-2), GHZ2 is an ideal formation site for the progenitors of today's globular clusters. The remarkable brightness of GHZ2 makes it a "Rosetta stone" for understanding the physics of galaxy formation within just 360 Myr after the Big Bang.