Grazian, A.A.GrazianGiallongo, E.E.GiallongoBoutsia, K.K.BoutsiaCristiani, S.S.CristianiVanzella, E.E.VanzellaScarlata, C.C.ScarlataSantini, P.P.SantiniPentericci, L.L.PentericciMerlin, E.E.MerlinMenci, N.N.MenciFontanot, F.F.FontanotFontana, A.A.FontanaFiore, F.F.FioreCivano, F.F.CivanoCastellano, M.M.CastellanoBrusa, M.M.BrusaBonchi, A.A.BonchiCarini, R.R.CariniCusano, F.F.CusanoFaccini, M.M.FacciniGarilli, B.B.GarilliMarchetti, A.A.MarchettiRossi, A.A.RossiSpeziali, R.R.Speziali2020-09-172020-09-172018-05-01https://hdl.handle.net/20.500.13025/5472Astronomy & Astrophysics, Volume 613, id.A44, 15 pp.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 at z > 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 at z ~ 4 down to an absolute magnitude of M_1450 ~ -23. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U - I color interval, with I 21-23, and 3.6 < z < 4.2. These AGNs are three magnitudes fainter than the typical SDSS QSOs (M_1450 ≲ -26) which are known to ionize their surrounding IGM at z ≳ 4. Methods. We acquired deep spectra of these faint AGNs with spectrographs available at the VLT, LBT, and Magellan telescopes, that is, FORS2, MODS1-2, and LDSS3, respectively. The emission in the Lyman Continuum region, close to 900 rest frame, has been detected with a signal to noise ratio of 10-120 for all 16 AGNs. The flux ratio between the 900 rest-frame region and 930 provides a robust estimate of the escape fraction of HI ionizing photons. Results. We have found that the Lyman Continuum escape fraction is between 44 and 100% for all the observed faint AGNs, with a mean value of 74% at 3.6 < z < 4.2 and - 25.1 ≲ M_1450 ≲ -23.3, in agreement with the value found in the literature for much brighter QSOs (M_1450 ≲ -26) at the same redshifts. The Lyman Continuum escape fraction of our faint AGNs does not show any dependence on the absolute luminosities or on the observed U - I colors of the objects. Assuming that the Lyman Continuum escape fraction remains close to ~ 75% down to M_1450 ~ - 18, we find that the AGN population can provide between 16 and 73% (depending on the adopted luminosity function) of the whole ionizing UV background at z ~ 4, measured through the Lyman forest. This contribution increases to 25-100% if other determinations of the ionizing UV background are adopted from the recent literature. Conclusions. Extrapolating these results to z ~ 5-7, there are possible indications that bright QSOs and faint AGNs can provide a significant contribution to the reionization of the Universe, if their space density is high at M_1450 ~ -23.quasars: generaldark agesreionizationfirst starsAstrophysics - Astrophysics of Galaxiesjournal article10.1051/0004-6361/201732385http://adsabs.harvard.edu/abs/2018A&A...613A..44G2018A&A...613A..44G