JADES - the small blue bump in GN-z11: insights into the nuclear region of a galaxy at z = 10.6

We report the detection of continuum excess in the rest-frame ultraviolet (UV) between 3000 and 3550 Ain 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 ×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 ∼3 ×10 4 K, significantly higher than the temperature of T e ( O 2 + ) = 1 . 36 ±0 . 13 ×10 4 K inferred from [O III ] λ4363 and [O III ] λ5007. 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 abo v e ∼1 / 3 solar is likely needed, which could be achiev ed via enrichment from Type-Ia superno vae, hyperno vae, or pair-instability supernovae.