The contribution of faint AGNs to the ionizing background at z ~ 4★

Abstract

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. <BR /> 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<SUB>1450</SUB> -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<SUB>1450</SUB> ≲-26) which are known to ionize their surrounding IGM at z ≳ 4. <BR /> 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 Å rest frame, has been detected with a signal to noise ratio of 10-120 for all 16 AGNs. The flux ratio between the 900 Å rest-frame region and 930 Å provides a robust estimate of the escape fraction of HI ionizing photons. <BR /> 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<SUB>1450</SUB> ≲-23.3, in agreement with the value found in the literature for much brighter QSOs (M<SUB>1450</SUB> ≲-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<SUB>1450</SUB> - 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. <BR /> 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<SUB>1450</SUB> -23. <P />Based on observations made at the Large Binocular Telescope (LBT) at Mt. Graham (Arizona, USA). Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 098.A-0862. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.