EMPRESS. XIV. Strong High-ionization Lines of Young Galaxies at z = 0–8: Ionizing Spectra Consistent with the Intermediate-mass Black Holes with M BH ∼ 103–106 M ⊙

We present ionizing spectra estimated at 13.6–100 eV for 10 dwarf galaxies with strong high-ionization lines of He II λ4686 and [Ne V] λ3426 ([Ne IV] λ2424) at z = 0 (z = 8) that are identified in our Keck/LRIS spectroscopy and the literature (the JWST Early Release Observations program). With the flux ratios of these high-ionization lines and >10 low-ionization lines of hydrogen, helium, oxygen, neon, and sulfur, we determine ionizing spectra consisting of stellar and nonthermal power-law radiation by photoionization modeling with free parameters of nebular properties, including metallicity and ionization parameter, canceling out abundance ratio differences. We find that all of the observed flux ratios are well reproduced by the photoionization models with the power-law index α _EUV of α _EUV ∼ (–1) ‑ 0 and the luminosity L _EUV of L _EUV ∼ 10⁴⁰–10⁴² erg s^‑1 at ∼55–100 eV for six galaxies, while four galaxies include large systematics in α _EUV caused by stellar radiation contamination. We then compare α _EUV and L _EUV of these six galaxies with those predicted by the black hole (BH) accretion disk models and find that α _EUV and L _EUV are similar to those of the intermediate-mass BHs (IMBHs) in BH accretion disk models, albeit with possibilities of the other scenarios. Confirming these results with a known IMBH having a mass M _BH of M _BH = 10^5.75 M _⊙, we find that four local galaxies and one z = 7.665 galaxy have ionizing spectra consistent with those of IMBHs with M _BH ∼ 10³–10⁵ M _⊙.