The escape fraction of ionizing photons from high redshift galaxies from data-constrained reionization models

Abstract: The escape fraction, f_{esc}, of ionizing photons from high-redshift galaxies is a key parameter to understand cosmic reionization and star formation history. Yet, in spite of many efforts, it remains largely uncertain. We propose a novel, semi-empirical approach based on a simultaneous match of the most recently determined Luminosity Functions (LF) of galaxies in the redshift range 6 \leq z \leq 10 with reionization models constrained by a large variety of experimental data. From this procedure we obtain the evolution of the best-fit values of f_{esc} along with their 2-sigma limits. We find that, averaged over the galaxy population, (i) the escape fraction increases from f_{esc} = 0.068_{-0.047}^{+0.054} at z=6 to f_{esc} = 0.179_{-0.132}^{+0.331} at z=8; (ii) at z=10 we can only put a lower limit of f_{esc} > 0.146. Thus, although errors are large, there is an indication of a 2.6 times increase of the average escape fraction from z=6 to z=8 which might partially release the "starving reionization" problem.