2024ApJ...966..170H

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 = 10^5.75M_⊙, we find that four local galaxies and one z = 7.665 galaxy have ionizing spectra consistent with those of IMBHs with M_BH ∼ 10³-10⁵M_⊙.