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∼ 10^3-10^6 M_⊙

We present ionizing spectra estimated at 13.6–100 eV for ten dwarf galaxies with strong high ionization lines of He iiλ4686 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 ERO 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 non-thermal power-law radiation by photoionization modeling with free parameters of nebular properties including metallicity and ionization parameter, cancelling out abundance ratio differences. We find that all of the observed flux ratios are well reproduced by the photoinization models with the power law index α_ EUV of α_ EUV∼ (-1)-0 and the luminosity L_ EUV of L_ EUV∼ 10^40-10^42 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 black holes (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∼ 10^3-10^5 M_⊙.