Metal-poor star formation at z > 6 with JWST: new insight into hard radiation fields and nitrogen enrichment on 20 pc scales

Abstract

Nearly a decade ago, we began to see indications that reionization-era galaxies power hard radiation fields rarely seen at lower redshift. Most striking were detections of nebular C IV emission in what appeared to be typical low-mass galaxies, requiring an ample supply of 48 eV photons to triply ionize carbon. We have obtained deep JWST/NIRSpec R = 1000 spectroscopy of the two z > 6 C IV-emitting galaxies known prior to JWST. Here, we present a rest-UV to optical spectrum of one of these two systems, the multiply-imaged z = 6.1 lensed galaxy RXCJ2248-ID. NIRCam imaging reveals two compact (<22 pc) clumps separated by 220 pc, with one comprising a dense concentration of massive stars (>10 400 M_⊙ yr^-1 kpc^-2) formed in a recent burst. We stack spectra of 3 images of the galaxy (J = 24.8-25.9), yielding a very deep spectrum providing a high-S/N template of strong emission line sources at z > 6. The spectrum reveals narrow high-ionization lines (He II, C IV, N IV]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([O III] EW = 2800 Å), albeit with weak emission from low-ionization transitions ([O III]/[O II] = 184). The electron density is found to be very high (6.4-31.0 × 10⁴ cm^-3) based on three UV transitions. The ionized gas is metal poor ($12+\log (\rm O/H)=7.43^{+0.17}_{-0.09}$), yet highly enriched in nitrogen ($\log (\rm N/O)=-0.39^{+0.11}_{-0.10}$). The spectrum appears broadly similar to that of GNz11 at z = 10.6, without showing the same AGN signatures. We suggest that the hard radiation field and rapid nitrogen enrichment may be a short-lived phase that many z > 6 galaxies go through as they undergo strong bursts of star formation. We comment on the potential link of such spectra to globular cluster formation.