Cosmic Reionization after Planck: Could Quasars Do It All?
  We assess a model of late cosmic reionization in which the ionizing
background radiation arises entirely from high redshift quasars and other
active galactic nuclei (AGNs). The low optical depth to Thomson scattering
reported by the Planck Collaboration pushes the redshift of instantaneous
reionization down to z=8.8^{+1.7}_{-1.4} and greatly reduces the need for
significant Lyman-continuum emission at very early times. We show that, if
recent claims of a numerous population of faint AGNs at z=4-6 are upheld, and
the high inferred AGN comoving emissivity at these epochs persists to higher,
z~10, redshifts, then active galaxies may drive the reionization of hydrogen
and helium with little contribution from normal star-forming galaxies. We
discuss an AGN-dominated scenario that satisfies a number of observational
constraints: the HI photoionization rate is relatively flat over the range
2<z<5, hydrogen gets fully reionized by z=5.7, and the integrated Thomson
scattering optical depth is tau=0.056, in agreement with measurements based on
the Lya opacity of the intergalactic medium (IGM) and cosmic microwave
background (CMB) polarization. It is a prediction of the model that helium gets
doubly reionized before redshift 4, the heat input from helium reionization
dominates the thermal balance of the IGM after hydrogen reionization, and z>5
AGNs provide a significant fraction of the unresolved X-ray background at 2
keV. Singly- and doubly-ionized helium contribute about 13% to tau, and the
HeIII volume fraction is already 50% when hydrogen becomes fully reionized.
