Rest-frame ultraviolet-to-optical spectral characteristics of extremely metal-poor and metal-free galaxies

Finding the first generation of galaxies in the early Universe is the greatest step forward for understanding galaxy formation and evolution. For strategic survey of such galaxies and interpretation of the obtained data, this paper presents an ultraviolet-to-optical spectral model of galaxies with a great care of the nebular emission. In particular, we present a machine-readable table of intensities of 119 nebular emission lines from Ly$\alpha$ to the rest-frame 1 $\mu$m as a function of metallicity from zero to the Solar one. Based on the spectral model, we present criteria of equivalent widths of Ly$\alpha$, He {\sc ii} $\lambda1640$, H$\alpha$, H$\beta$, [O {\sc iii}] $\lambda5007$ to select extremely metal-poor and metal-free galaxies although these criteria have uncertainty caused by the Lyman continuum escape fraction and the star formation duration. We also present criteria of broad-band colours which will be useful to select candidates for spectroscopic follow-up from drop-out galaxies. We propose the line intensity ratio of [O {\sc iii}] $\lambda5007$ to H$\beta$ $<0.1$ as the most robust criterion for $<1/1000$ of the Solar metallicity. This ratio of a galaxy with a few $M_\odot$ yr$^{-1}$ at $z\sim8$ is detectable by spectroscopy with the James Webb Space Telescope within a reasonable exposure time.