We revisit the classical view that quasar contribution to the reionization of hydrogen is unimportant. When secondary ionizations are taken into account, in many plausible scenarios for the formation and growth of supermassive black holes quasars contribute substantially or even dominantly at z ≳ 8, although their contribution generally falls below that of star-forming galaxies by z = 6. Theoretical models that guide the design of the first generation of redshifted 21 cm experiments must, therefore, substantially account for the quasar contribution in order to be even qualitatively accurate.
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Volonteri et al. (2009) studied this question.