The Spectral Evolution of the First Galaxies. III. Simulated James Webb Space Telescope Spectra of Reionization-epoch Galaxies with Lyman-continuum Leakage

Using four different suites of cosmological simulations, we generate synthetic spectra for galaxies with different Lyman-continuum ESCape fractions (f_ESC) at redshifts z ≈ 7-9, in the rest-frame wavelength range relevant for the James Webb Space TelESCope (JWST) NIRSpec instrument. By investigating the effects of realistic star formation histories and metallicity distributions on the EW(Hβ)-β diagram (previously proposed as a tool for identifying galaxies with very high f_ESC), we find that neither of these effects are likely to jeopardize the identification of galaxies with extreme Lyman-continuum leakage. Based on our models, we expect that essentially all z ≈ 7-9 galaxies that exhibit rest-frame EW(Hβ) ≲ 30 Å to have f_ESC > 0.5.. Incorrect assumptions concerning the ionizing fluxes of stellar populations or the dust properties of z > 6 galaxies can in principle bias the selection, but substantial model deficiencies of this type should at the same time be evident from offsets in the observed distribution of z > 6 galaxies in the EW(Hβ)-β diagram compared to the simulated distribution. Such offsets would thereby allow JWST/NIRSpec measurements of these observables to serve as input for further model refinement.