COSMIC REIONIZATION AFTER PLANCK: COULD QUASARS DO IT ALL?

TL;DRAbstract

We assess a model of late cosmic reionization in which the ionizing\nbackground radiation arises entirely from high redshift quasars and other\nactive galactic nuclei (AGNs). The low optical depth to Thomson scattering\nreported by the Planck Collaboration pushes the redshift of instantaneous\nreionization down to z=8.8^{+1.7}_{-1.4} and greatly reduces the need for\nsignificant Lyman-continuum emission at very early times. We show that, if\nrecent claims of a numerous population of faint AGNs at z=4-6 are upheld, and\nthe high inferred AGN comoving emissivity at these epochs persists to higher,\nz~10, redshifts, then active galaxies may drive the reionization of hydrogen\nand helium with little contribution from normal star-forming galaxies. We\ndiscuss an AGN-dominated scenario that satisfies a number of observational\nconstraints: the HI photoionization rate is relatively flat over the range\n2<z<5, hydrogen gets fully reionized by z=5.7, and the integrated Thomson\nscattering

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