Deep Rest-UV JWST/NIRSpec Spectroscopy of Early Galaxies: The Demographics of C iv and N-emitters in the Reionization Era

Abstract

JWST has recently discovered a subset of reionization era galaxies with ionized gas that is metal-poor in oxygen and carbon but heavily enriched in nitrogen. This abundance pattern is almost never seen in lower-redshift galaxies but is commonly observed in globular cluster stars. We have recently demonstrated that this peculiar abundance pattern appears in a compact (≃20 pc) metal-poor galaxy undergoing a strong burst of star formation. This galaxy was originally selected based on strong C iv emission, indicating a hard radiation field rarely seen locally. In this paper, we present JWST/NIRSpec observations of another reionization-era galaxy known to power strong C iv emission, the z = 7.04 gravitationally lensed galaxy A1703-zd6. The emission-line spectrum reveals this is a metal-poor galaxy ( $12+\mathrm{log}({\rm{O}}/{\rm{H}})=7.47\pm 0.19$ ) dominated by a young stellar population ( $1.{6}_{-0.4}^{+0.5}$ Myr) that powers a very hard ionizing spectrum (C iv equivalent width, EW = 19.4 Å, He ii EW = 2.2 Å). The interstellar medium is highly enriched in nitrogen ( $\mathrm{log}({\rm{N}}/{\rm{O}})=-0.6$ ) with very high electron densities (8–19 × 10 ^4 cm ^−3 ) and extreme ionization conditions rarely seen at lower redshift. We also find intense CIV emission (EW ≳ 20 Å) in two new z ≳ 6 metal-poor galaxies. To put these results in context, we search for UV line emission in a sample of 737 z ≳ 4 galaxies with NIRSpec spectra, establishing that 40%(30%) of systems with [O iii ]+H β EW > 2000 Å have N iv ] (C iv ) detections with EW > 5 Å(> 10 Å). These results suggest high N/O ratios, and hard ionizing sources appear in a brief phase following a burst of star formation in compact high-density stellar complexes.

Keywords

• Galaxy abundances
• Galaxy chemical evolution
• High-redshift galaxies
• Galactic and extragalactic astronomy
• Starburst galaxies