Search for High-Ionization Nebular Emission (SHINE). I. A Systematically Selected [Ne V] Sample at z > 3 with JWST/NIRSpec PRISM
We conduct the first systematic Search for High-Ionization Nebular Emission (SHINE) using [Ne V]$\lambda3426$ in JWST/NIRSpec PRISM spectroscopy at $z>3$. From more than 9,000 galaxies, we identify 25 [Ne V] emitters spanning $z=3.059$-$9.444$ and $\log_{10}(L_{\rm [Ne\,V]}/erg\,s^{-1})=40.70$-$42.86$. Their [Ne V] luminosities overlap with those of local [Ne V]-selected active galactic nuclei (AGNs) and exceed those of local metal-poor [Ne V] emitters, although such low-luminosity systems would fall below our sensitivity. The population is diverse, spanning compact and extended morphologies and a broad range of continuum properties and stellar masses, including very low-mass hosts. It includes sources with broad Balmer emission as well as others whose higher-resolution spectra do not require a broad component. The [Ne V] emitters overlap only partly with conventional AGN diagnostics: most do not satisfy conservative high-redshift AGN criteria based on strong rest-optical narrow-line ratios, and only a minority have adopted broad-line classifications. [Ne V] upper limits for independently selected broad-line AGNs and little red dots are too shallow to establish a population-wide [Ne V] deficit. Six sources have secure Chandra counterparts, showing that X-ray weakness is not universal among luminous [Ne V] emitters. Strong [Ne V]/[Ne III] emission is associated with redder ultraviolet slopes and stronger Balmer breaks, but not with UV luminosity, possibly linking strong high-ionization emission to recent changes in star formation. The observed incidence of luminous [Ne V] emission shows no significant evolution over $z>3$, despite an increasing robust [Ne III] detection fraction. The high [Ne V] luminosities and hard line ratios favor black-hole accretion as the dominant power source, establishing [Ne V] emission as a complementary probe of early black-hole growth.