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We present ionizing spectra estimated at 13.6--100 eV for ten dwarf galaxieswith strong high ionization lines of He {\sc {ii}}$\lambda$4686 and [Ne{\sc{v}}]$\lambda$3426 ([Ne {\sc{iv}}]$\lambda$2424) at $z=0$ ($z=8$) that areidentified in our Keck/LRIS spectroscopy and the literature (the JWST EROprogram). With the flux ratios of these high ionization lines and $>10$low-ionization lines of hydrogen, helium, oxygen, neon, and sulfur, wedetermine ionizing spectra consisting of stellar and non-thermal power-lawradiation by photoionization modeling with free parameters of nebularproperties including metallicity and ionization parameter, cancelling outabundance ratio differences. We find that all of the observed flux ratios arewell reproduced by the photoinization models with the power law index$\alpha_{\rm EUV}$ of $\alpha_{\rm EUV}\sim (-1)-0$ and the luminosity $L_{\rmEUV}$ of $L_{\rm EUV}\sim 10^{40}-10^{42}$ erg s$^{-1}$ at $\sim 55-100$ eV forsix galaxies, while four galaxies include large systematics in $\alpha_{\rmEUV}$ caused by stellar radiation contamination. We then compare $\alpha_{\rmEUV}$ and $L_{\rm EUV}$ of these six galaxies with those predicted by the blackhole (BH) accretion disk models, and find that these galaxies have moderatelysoft/luminous ionizing spectra whose $\alpha_{\rm EUV}$ and $L_{\rm EUV}$ aresimilar to those of the intermediate mass black holes (IMBHs) in BH accretiondisk models. Confirming these results with a known IMBH having a mass $M_{\rmBH}$ of $M_{\rm BH}=10^{5.75} \ M_\odot$, we find that four local galaxies andone $z=7.665$ galaxy have ionizing spectra consistent with those of IMBHs with$M_{\rm BH} \sim 10^3-10^5 \ M_\odot$.

EMPRESS. XIV. Strong High Ionization Lines of Young Galaxies at $z=0-8$:  Ionizing Spectra Consistent with the Intermediate Mass Black Holes with  $M_{\rm BH}\sim 10^3-10^6\ M_\odot$