JADES NIRSpec Spectroscopy of GN-z11: Lyman-$α$ emission and possible enhanced nitrogen abundance in a $z=10.60$ luminous galaxy

TL;DR

JWST/NIRSpec provides a robust spectroscopic redshift of $z=10.6034$ for GN-z11, one of the most luminous known galaxies at $z>10$, and reveals a rich rest-frame UV–optical emission-line spectrum enabling precise ISM diagnostics. The galaxy exhibits a high ionization parameter, $\log U \approx -2$, with nebular metallicity $Z_{\mathrm{neb}} \approx 0.1\,Z_\odot$ and unusually strong N III]/N IV] lines suggesting enhanced nitrogen relative to oxygen, all while showing low dust attenuation and SFR of order $20$–$30\,M_\odot\,\mathrm{yr}^{-1}$. A Lyman-α line is detected at $z>9$ with a velocity offset ~$555\,\mathrm{km\,s^{-1}}$ and spatially extended emission, consistent with backscattering in a galactic outflow within a largely neutral IGM. Together, these results illuminate extreme ISM conditions and rapid chemical enrichment in the first few hundred million years after the Big Bang, demonstrating the transformative power of JWST for probing the early cosmic epoch.

Abstract

We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous candidate $z>10$ Lyman break galaxy in the GOODS-North field with $M_{UV}=-21.5$. We derive a redshift of $z=10.603$ (lower than previous determinations) based on multiple emission lines in our low and medium resolution spectra over $0.8-5.3 μ$m. We significantly detect the continuum and measure a blue rest-UV spectral slope of $β=-2.4$. Remarkably, we see spatially-extended Lyman-$α$ in emission (despite the highly-neutral IGM expected at this early epoch), offset 555 km s$^{-1}$ redward of the systemic redshift. From our measurements of collisionally-excited lines of both low- and high-ionization (including [O II]$\lambda3727$, [Ne III]$λ3869$ and C III]$\lambda1909$) we infer a high ionization parameter ($\log U\sim -2$). We detect the rarely-seen N IV]$\lambda1486$ and N III]$\lambda1748$ lines in both our low and medium resolution spectra, with other high ionization lines seen in the low resolution spectrum such as He II (blended with O III]) and C IV (with a possible P-Cygni profile). Based on the observed rest-UV line ratios, we cannot conclusively rule out photoionization from AGN, although the high C III]/He II and N III]/He II ratios are compatible with a star-formation explanation. If the observed emission lines are powered by star formation, then the strong N III]$\lambda1748$ observed may imply an unusually high $N/O$ abundance. Balmer emission lines (H$γ$, H$δ$) are also detected, and if powered by star formation rather than an AGN we infer a star formation rate of $\sim 20-30 M_{\odot} yr^{-1}$ (depending on the IMF) and low dust attenuation. Our NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme properties seen 430 Myr after the Big Bang.

Figures (12)

• Figure 1: The NIRCam F200W image of GN-z11 (see Tacchella2023) with the NIRSpec microshutters overlaid for the four different pointings. The green rectangles denote the illuminated region of each microshutter ($0\farcs2 \times 0\farcs 46$). North is up and East to the left, and the image is 3 arcsec on the side. The flux density units on the colour bar are MJy/sr.
• Figure 2: 2D (top) and 1D (bottom) spectra of GN-z11 using PRISM/CLEAR configuration of NIRSpec. The 1D spectrum has been extracted using a 3 pixel wide aperture that leads to improved S/N in this highly compact object. Prominent emission lines present in the spectra are marked. The signal to noise ratio (SNR) of the continuum is high and the emission lines are clearly seen in both the 1D and 2D spectra.
• Figure 3: Gallery of the most prominent emission lines seen in the spectrum GN-z11 from the Medium resolution (R1000) gratings using a 3 pixel 1D spectral extraction.
• Figure 4: Measured C iii]/He ii vs C iii]/C iv ratios for GN-z11 shown along with predictions from photoionization due to AGN (circles) and star-formation (stars) from Feltre2016 and Gutkin2016 considering in the range of $Z/Z_\odot = 0.066 - 0.131$ and gas densities in the range $\log(n_H)/\textrm{cm}^{-3} = 2 - 4$. Based on the observed line ratios neither photoionization from AGN or star-formation alone can conclusively explain the observations, placing GNz11 right between the model predictions from the two.
• Figure 5: Velocity offset of the Ly$\alpha$ emission line (blue solid line) compared with the H $\gamma$ line (green dashed line). The Ly$\alpha$ line is redshifted by $555$ km s$^{-1}$ compared to the redshift derived from other emission lines in the spectrum.