JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift

Mengtao Tang, Daniel P. Stark, Adèle Plat, Anna Feltre, Harley Katz, Peter Senchyna, Charlotte A. Mason, Lily Whitler, Zuyi Chen, Michael W. Topping

First listed 2025-05-09 | Last updated 2025-08-18

Abstract

JWST spectroscopy has built large emission line samples at $z\gtrsim4$, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with AGN photoionization. While this may indicate a weaker UV ionizing spectrum in many $z>4$ AGNs or obscuration from dense neutral gas and dust, the complete picture remains unclear owing to the small number of deep rest-UV spectra. Here we characterize the strength of high ionization lines in $53$ new galaxies observed with NIRSpec $R=2700$ grating spectroscopy. We present new detections of narrow NV$\lambda1240$ in two galaxies. One is a previously-confirmed $z=6.98$ Little Red Dot (LRD) with broad H$β$, and the other is a $z=8.72$ galaxy with a narrow line spectrum. Neither source exhibits CIV or HeII emission, indicating large NV/CIV and NV/HeII ratios that may reflect a combination of nitrogen-enhancement and resonant scattering effects. We investigate the incidence of narrow high ionization lines in a large database of $851$ NIRSpec grating spectra, and we separately quantify the fraction of LRDs with narrow high ionization UV emission lines. Our results likely suggest that hard radiation fields are indeed present in a small subset of LRDs ($12.5^{+23.7}_{-10.4}\%$) and UV-selected galaxies ($2.2^{+1.7}_{-1.0}\%$) at $z>4$. The identification of narrow high ionization lines in the population of LRDs with strong Balmer absorption suggests the dense neutral hydrogen gas may not uniformly cover the nucleus. The strong NV (coupled with weak CIV and HeII) suggests that efforts to identify high ionization lines should extend down in wavelength to the NV doublet.

Short digest

Deep NIRSpec R=2700 spectroscopy of 53 new z>4 galaxies, plus a census of 851 archival spectra, is used to track high-ionization UV lines. The team reports narrow NV λ1240 in two systems—CEERS‑7902 (z=6.98; an LRD with broad Hβ) and CEERS‑1025 (z=8.72; narrow-line)—while CIV and He II are absent, yielding extreme NV/CIV and NV/He II. The inferred incidence of narrow high-ionization UV lines is 12.5(+23.7/−10.4)% in LRDs and 2.2(+1.7/−1.0)% in UV-selected galaxies at z>4, implying hard radiation fields in a small subset and that dense H I may not fully cover some LRD nuclei. The line ratios and non-detections argue for extending searches down to the NV doublet, though statistics remain limited by the number of deep rest-UV spectra.

Key figures to inspect

• Figure 2 (CEERS‑1025 G140H/G395M): Inspect the resolved NV doublet at the systemic redshift and verify the strong upper limits on CIV and He II; note the tentative [O III] detection anchoring zsys and the narrow-line widths.
• Figure 3 (NV/CIV vs NV/He II): See where CEERS‑1025 and CEERS‑7902 land relative to low‑z AGN loci—both occupy the high‑NV quadrant, supporting nitrogen enhancement and/or resonant scattering rather than typical CIV/He II‑bright AGN.
• Figure 4 ([O III]/[O II] vs [O III] EW): Compare the two NV emitters to CIV emitters and the [Ne V] source GN‑42437 to gauge ionization parameter and hardness; note whether the NV emitters align with extreme [O III] EW trends.
• Figure 1 (SEDs/prism context): Use the BEAGLE fit for CEERS‑1025 and the prism spectra for LRDs CEERS‑7902 and CEERS‑10444 to understand continuum shapes, photometric constraints, and LRD SED context for the UV line analysis.