摘要：Context.Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in observational cosmology. Bright quasi-stellar objects (QSOs) are known to ionize their surrounding neighborhood, but they are too few to ensure the required HI ionizing background. A significant contribution by faint active galactic nuclei (AGNs), however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei atz> 4.Aims.This work is part of a long-term project aimed at measuring the Lyman Continuum escape fraction for a large sample of AGNs atz~ 4 down to an absolute magnitude ofM1450~ −23. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broadU−Icolor interval, withI~ 21–23, and 3.6
关键词：Key wordsenquasars: generaldark ages, reionization, first stars
