2025MNRAS.541.2134J

We report the detection of continuum excess in the rest-frame ultraviolet (UV) between 3000 and 3550 Å in the JWST/Near Infrared Spectrograph (NIRSpec) spectrum of GN-z11, a luminous galaxy $z=10.603$. The shape of the continuum excess resembles a Balmer continuum, but has a break around 3546 Å. The fitting result of this excess depends on the assumed origin of the continuum. If the continuum of GN-z11 is dominated by a stellar population with a small Balmer break, the apparent blueshift of the Balmer continuum is not significant and the best-fitting Balmer continuum model indicates a temperature of $T_e = 1.78^{+0.25}_{-0.21}\times 10^4$ K. In contrast, if the continuum is dominated by active galactic nucleus emission, a nebular continuum model cannot fit the spectrum properly. The absence of the Balmer jump indicates an electron temperature of $\sim 3\times 10^4$ K, significantly higher than the temperature of $T_{e}({\rm O^{2+}}) = 1.36\pm 0.13\times 10^{4}$ K inferred from [O III]$\lambda 4363$ and [O III]$\lambda 5007$. The temperature difference can result from mixing of different ionized regions: the Balmer emission mainly arises from dense and hot clouds in the broad-line region, whereas the forbidden lines originate from less dense and colder gas. An alternative explanation for the observed continuum excess is the Fe II emission, which shows a characteristic jump blueward of the Balmer limit as previously seen in the spectra of many lower redshift quasars. Through comparisons with CLOUDY models, we show an Fe abundance above $\sim 1/3$ solar is likely needed, which could be achieved via enrichment from Type-Ia supernovae, hypernovae, or pair-instability supernovae.