On the contribution of active galactic nuclei to reionization (Research
  Note)
  The electron scattering optical depth constraints on reionization suggest
that there may be other sources that contribute to the ionization of hydrogen
aside from observable star forming galaxies. Often the calculated value of the
electron scattering optical depth, \tau_{es}, falls below the measurements
derived from observations of the CMB or an assumption about non-observable
sources must be made in order to reach agreement. Here, we calculate the
hydrogen ionization fraction as a function of redshift and the electron
scattering optical depth from both galaxies and active galactic nuclei (AGN)
factoring in the secondary collisional ionizations from the AGN X-ray emission.
In this paper we use the most current determination of the evolving hard X-ray
luminosity function and extrapolate its evolution beyond z = 6. The AGN
spectral energy distributions (SEDs) include both UV and X-ray ionizing
photons. To search for the largest possible effect, all AGN are assumed to have
\lambda_{Edd} = 1.0 and be completely unobscured. The results show that AGNs
produce a perturbative effect on the reionization of hydrogen and remains in
agreement with current constraints. Our calculations find the epoch of
reionization still ends at z \approx 6 and only increases the electron
scattering optical depth by ~2.3% under the most optimal conditions. This can
only be moderately increased by assuming a constant black hole mass of M_{BH} =
10^(5) solar masses. As a result, we conclude that there is a need for other
sources beyond observable galaxies and AGNs that contribute to the reionization
of hydrogen at z > 6.
