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 towards understanding galaxy formation and evolution. For a strategic survey of such galaxies and the 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 machinereadable 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 II lambda 1640, H alpha, H beta and [OIII]lambda 5007 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 [OIII]lambda 5007 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(circle dot)yr(-1) at z similar to 8 is detectable by spectroscopy with the James Webb Space Telescope within a reasonable exposure time.