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 (fesc) that escape from z>6 galaxies is an
important parameter when assessing the role of these objects in the
reionization of the Universe, but the opacity of the intergalactic medium
precludes a direct measurement of fesc for individual galaxies at these epochs.
We argue, that since fesc regulates the impact of nebular emission on the
spectra of galaxies, it should nonetheless be possible to indirectly probe fesc
well into the reionization epoch. As a first step, we demonstrate that by
combining measurements of the rest-frame UV slope beta with the equivalent
width of the Hb emission line, galaxies with very high Lyman continuum escape
fractions (fesc>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 the dust properties of these galaxies.
