The [Ne V] AGN Diagnostic in SDSS-IV/eBOSS: An Anticorrelation Between Ionization State and AGN Luminosity in Low-Redshift Coronal-Line Galaxies

Owen S. Matthews Acuña, Christy A. Tremonti, Nikko J. Cleri, Kyle B. Westfall, Bee R. Erena, Jacob B. Stimac, Britt Lundgren, Drake Miller, Aleksandar M. Diamond-Stanic

First listed 2026-08-24 | Last updated 2026-08-21

Abstract

Traditional narrow-line diagnostics fail at high redshift, where low metallicity drives star-forming galaxies and Active Galactic Nuclei (AGN) into overlapping regions of the Baldwin-Phillips-Terlevich (BPT) diagram. Coronal lines, with ionization potentials exceeding 100 eV, offer a robust alternative: they cannot be produced by normal stellar populations, arise almost exclusively from AGN or fast shocks, and are insensitive to abundance evolution. Among these, the [Ne V] doublet is the brightest optical coronal line tracer of AGN activity. Existing coronal line catalogs contain only about 1750 objects, too few to statistically assess the completeness and purity of [Ne V]-selected AGN samples. We identify 33,817 galaxies with [Ne V] S/N > 5 at z = 0.147-1.12 in the Sloan Digital Sky Survey IV extended Baryon Oscillations Spectroscopic Survey (SDSS-IV/eBOSS), exceeding the combined literature total by more than an order of magnitude. [Ne V] preferentially selects rapidly accreting, relatively unobscured AGN; only 41.5% of BPT AGN show detectable emission. By stacking spectra that lack [Ne V] on a grid of black hole mass and [O III] luminosity, we recover [Ne V] in the majority of bins, implying that non-detections reflect limited survey sensitivity rather than a deficit of coronal emission. We find [Ne V]/[Ne III] is only weakly correlated with [O III]/[O II]. Galaxies with high [Ne V]/[Ne III] for their [O III]/[O II] ratio have lower Eddington ratios and are more likely classified as LIERs or Composites. The [Ne V]/[O III] ratio shows a strong anticorrelation with [O III] luminosity, with the most extreme coronal-line strengths found among the lowest-luminosity AGN, in analogy with the accretion states of stellar-mass black holes. Comparison with a sample of z=2-9 [Ne V]-detected galaxies suggests some, but not all, high-redshift AGN follow the same relations.

Short digest

Matthews Acuña et al. assemble 33,817 [Ne V]-detected galaxies at z=0.147–1.12 from SDSS-IV/eBOSS, creating by far the largest optical coronal-line AGN sample and testing how this diagnostic selects accreting black holes. [Ne V] preferentially identifies luminous, rapidly accreting, relatively unobscured BPT AGN, but stacks recover the line in 92.6% of black-hole-mass–[O III]-luminosity bins with at least five non-coronal AGN, showing that many individual non-detections are a sensitivity limitation rather than an absence of coronal emission. The key physical result is a strong decline of [Ne V]/[O III] with [O III] luminosity, while deviations in [Ne V]/[Ne III] at fixed [O III]/[O II] track lower Eddington ratios and more LIER/composite-like classifications, motivating an analogy to a continuum of low/hard and high/soft accretion states that can now be tested in z=2–9 [Ne V] sources.

Key figures to inspect

• Figure 1. This establishes the survey-scale advance: the 33,817-object eBOSS [Ne V] catalog exceeds the combined six major prior coronal-line catalogs by more than an order of magnitude across their overlapping redshift range.
• Figure 6. The black-hole-mass–[O III]-luminosity stacking experiment directly supports the paper’s selection-function conclusion, recovering [Ne V] in 150 of 162 sufficiently populated bins and showing that most individual non-detections are driven by depth rather than missing coronal emission.
• Figure 14. This comparison of High53 and Low53 galaxies shows that Eddington ratio separates the off-locus [Ne V]/[Ne III] populations, tying the ionization-state diagnostic to accretion state and to the paper’s LIER/composite interpretation.
• Figure 15. This is the synthesis figure for the central luminosity result: it places individual eBOSS galaxies, stacks, and z=2–9 literature AGN on the line-ratio–luminosity relations, including the strong [Ne V]/[O III] anticorrelation and its possible extension to the early universe.