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

Michael W. Topping, Daniel P. Stark, Peter Senchyna, Adele Plat, Adi Zitrin, Ryan Endsley, Stéphane Charlot, Lukas J. Furtak, Michael V. Maseda, Renske Smit, Ramesh Mainali, Jacopo Chevallard, Stephen Molyneux, Jane R. Rigby

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


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(O/H) = 7.43+−00 .0917), yet highly enriched in nitrogen (log(N/O) = −0.39+−00.1011). 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.

Original languageEnglish
Pages (from-to)3301-3322
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - 1 Apr 2024


  • galaxies: ISM
  • galaxies: evolution
  • galaxies: high-redshift

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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