The spectral evolution of the first galaxies. II. Spectral signatures of lyman continuum leakage from galaxies in the reionization epoch

The fraction of ionizing photons that escape (f _esc) from z ≳ 6 galaxies is an important parameter for assessing the role of these objects in the reionization of the universe, but the opacity of the intergalactic medium precludes a direct measurement of f _esc for individual galaxies at these epochs. We argue that since f _esc regulates the impact of nebular emission on the spectra of galaxies, it should nonetheless be possible to indirectly probe f _esc well into the reionization epoch. As a first step, we demonstrate that by combining measurements of the rest-frame UV slope β with the equivalent width of the Hβ emission line, galaxies with very high Lyman continuum escape fractions (f _esc ≥ 0.5) should be identifiable up to z = 9 through spectroscopy with the upcoming James Webb Space Telescope (JWST). By targeting strongly lensed galaxies behind low-redshift galaxy clusters, JWST spectra of sufficiently good quality can be obtained for M 1500 ≲ -16.0 galaxies at z = 7 and for M 1500 ≲ -17.5 galaxies at z = 9. Dust-obscured star formation may complicate the analysis, but supporting observations with ALMA or the planned SPICA mission may provide useful constraints on this effect.