Nitrogen enhancements 440 Myr after the Big Bang: super-solar N/O, a tidal disruption event or a dense stellar cluster in GN-z11?

Abstract

Recent observations of GN-z11 with JWST/NIRSpec revealed numerous oxygen, carbon, nitrogen, and helium emission lines at $z=10.6$. Using the measured line fluxes, we derive abundance ratios of individual elements within the interstellar medium (ISM) of this super-luminous galaxy. Driven by the unusually-bright NIII] $\lambda$1750 and NIV] $\lambda$1486 emission lines (and by comparison faint OIII] $\lambda\lambda$1660, 1666 lines), our fiducial model prefers log(N/O)>-0.25, greater than four times solar and in stark contrast to lower-redshift star-forming galaxies. The derived log(C/O)>-0.78, ($\approx$30 % solar) is also elevated with respect to galaxies of similar metallicity (12+log(O/H)$\approx7.82$), although less at odds with lower-redshift measurements. Given the long timescale typically expected to enrich nitrogen with stellar winds, traditional scenarios require a very fine-tuned formation history to reproduce such an elevated N/O. We find no compelling evidence that nitrogen enhancement in GN-z11 can be explained by enrichment from metal-free Population III stars. Interestingly, yields from runaway stellar collisions in a dense stellar cluster or a tidal disruption event provide promising solutions to give rise to these unusual emission lines at $z=10.6$, and explain the resemblance between GN-z11 and a nitrogen-loud quasar. These recent observations showcase the new frontier opened by JWST to constrain galactic enrichment and stellar evolution within 440 Myr of the Big Bang.