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JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift
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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$\beta$, 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.
Forward citations
Cited by 6 Pith papers
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