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 CIV 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 CIV-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 (<22pc) clumps separated by 220pc, with one comprising a dense concentration of massive stars (>10, 400M⊙/yr/kpc2) 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 (HeII, CIV, NIV]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([OIII] EW=2800Å), albeit with weak emission from low-ionization transitions ([OIII]/[OII]=184). The electron density is found to be very high (6.4 − 31.0 × 104cm−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.

No Top-heavy Stellar Initial Mass Function Needed: The Ionizing Radiation of GS9422 Can Be Powered by a Mixture of an Active Galactic Nucleus and Stars

“…show super-solar N/O (e.g., Bunker et al 2023b;Castellano et al 2024;Topping et al 2024) The high rate of hard ionizing photons can be explained by an AGN. AGN ionizing spectra are conventionally assumed to have a spectral index of 1.2 α AGN 2 in F ν (e.g., Groves et al 2004;Feltre et al 2016).…”

Resolving the Mechanical and Radiative Feedback in J1044+0353 with Keck Cosmic Web Imager Spectral Mapping

“…Normal stellar populations, as described by Binary Population and Spectral Synthesis v2.1 (BPASS) models (Eldridge et al 2017), for example, do not produce enough 54.4 eV photons to explain the He II λ1640 emission from J1044+0353 (Berg et al 2019(Berg et al , 2021Olivier et al 2022). This high-energy ionizing photon production problem has been observed in other local galaxies (Shirazi & Brinchmann 2012;Jaskot & Oey 2013;Stasińska et al 2015;Senchyna et al 2017;Schaerer et al 2019;Mingozzi et al 2022Mingozzi et al , 2023 and is a common feature of reionization-era galaxies (Stark et al 2015;Mainali et al 2017;Bunker et al 2023;Senchyna et al 2023;Topping et al 2024). The damped Lyα profile in the same FUV spectrum of J1044 +0353 (Hu et al 2023), however, indicates a low f esc value along our sight line.…”

CECILIA: Direct O, N, S, and Ar Abundances in Q2343-D40, a Galaxy at z ∼ 3

“…However, with the NIR capabilities of the James Webb Space Telescope (JWST), direct T e measurements and abundances are now readily accessible in high-z galaxies (e.g., Arellano-Córdova et al 2022a;Schaerer et al 2022;Sanders et al 2024;Laseter et al 2024;Trump et al 2023;Topping et al 2024;Welch et al 2024). While these studies have started to uncover the chemical abundance patterns in the early Universe, the current scope of high-z chemical abundances is still fairly limited.…”