The influence of QSO emission-line velocity shifts on estimates of the intergalactic background intensity

Abstract

Observations of the inverse effect indicate that the observed number of QSOs cannot provide sufficient radiation to reionize the intergalactic medium. We reanalyze the data set of one such study and show that bias in the measurement of QSO redshifts can account for at least a proportion of the discrepancy between the observed and predicted ionizing background when current QSO luminosity evolution models are used. The exact value of the shortfall depends on the assumed opacity due to intervening absorption systems, but for one optically thick model our reassessment indicates that the apparent discrepancy between the observed ionizing background strength and that predicted from 'QSO-only' models is halved. This result places ionization models which solely involve QSO ionizing radiation within a factor of about 4 of supplying all the photons required by the observations and may be within a factor of 2 if the opacity of the universe has been overestimated in current models. When account is taken of the incompleteness of the QSO luminosity function at high redshift, it is quite possible that the bulk of the intergalactic ionizing radiation is provided by active galactic nuclei.