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

Abstract JWST spectroscopy has built large emission line samples at z ≳ 4, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with active galactic nucleus photoionization. While this may indicate a weaker UV ionizing spectrum in many z > 4 active galactic nuclei 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 N v λ 1240 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 C iv or He ii emission, indicating large N v /C iv and N v /He ii 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 − 10.4 + 23.7 % ) and UV-selected galaxies ( 2 . 2 − 1.0 + 1.7 % ) at z > 4. The identification of narrow high-ionization lines in the population of LRDs with strong Balmer absorption suggests that the dense neutral hydrogen gas may not uniformly cover the nucleus. The strong N v (coupled with weak C iv and He ii ) suggests that efforts to identify high-ionization lines should extend down in wavelength to the N v doublet.