Abstract
Thomson optical depth measurements from Planck provide new insights into the
reionization of the universe. To obtain new model-independent constraints on
the properties of the ionizing sources, we determine the empirical evolution of
the ionising background. We use a simple two-parameter model to map out the
evolution in this background at z>~6 from the new Planck optical depth tau
measurements and from the constraints provided by quasar absorption spectra and
the prevalence of Ly-alpha emission in z~7-8 galaxies. We find the redshift
evolution in the ionising background N_ion required by the observations to be
dlog_10 N_ion/dz(z=8)=-0.19_-0.11^+0.09, largely independent of the
assumed clumping factor C_HII and entirely independent of the identity of the
ionizing sources. The trend in N_ion is well-matched by the evolution of the
galaxy UV-luminosity density (dlog_10 rho_UV/dz=-0.11+/-0.04) to a
magnitude limit >~-13 mag, suggesting that galaxies are the sources that drive
the reionization of the universe. The role of galaxies is further strengthened
by the conversion from the UV luminosity density to N_ion(z) being possible
for physically plausible values of the escape fraction f_esc, the
Lyman-continuum photon production efficiency xi_ion, and faint-end cut-off
M_lim to the LF. Lastly, we use the inferred evolution in the ionizing
background to estimate the z~10 UV luminosity density, finding this luminosity
density to be 12_-7^+21x lower than at z~6, consistent with current
measurements at z~10. Quasars/AGN appear to match neither the redshift
evolution nor normalization of the ionizing background. This new approach of
contrasting the inferred evolution of the ionising background with that of the
galaxy UV luminosity density adds to the growing observational evidence that
galaxies are the sources that drive the reionization of the universe.
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