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

Topping, MW ORCID: 0000-0001-8426-1141, Stark, DP, Senchyna, P ORCID: 0000-0002-9132-6561, Plat, A, Zitrin, A ORCID: 0000-0002-0350-4488, Endsley, R ORCID: 0000-0003-4564-2771, Charlot, S ORCID: 0000-0003-3458-2275, Furtak, LJ ORCID: 0000-0001-6278-032X, Maseda, MV ORCID: 0000-0003-0695-4414, Smit, R ORCID: 0000-0001-8034-7802, Mainali, R, Chevallard, J ORCID: 0000-0002-7636-0534, Molyneux, S ORCID: 0000-0003-3596-622X and Rigby, JR (2024) Metal-poor star formation at z > 6 with JWST: new insight into hard radiation fields and nitrogen enrichment on 20 pc scales. Monthly Notices of the Royal Astronomical Society, 529 (4). pp. 3301-3322. ISSN 0035-8711

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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(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.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; Astronomy & Astrophysics
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	4 March 2024
Date of first compliant Open Access:	20 August 2024
Date Deposited:	20 Aug 2024 11:03
Last Modified:	04 Jul 2025 09:45
DOI or ID number:	10.1093/mnras/stae682
URI:	https://researchonline.ljmu.ac.uk/id/eprint/23991

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