The Galaxy Luminosity Function during the Reionization Epoch
  The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field
started investigating the properties of galaxies during the reionization epoch.
To interpret these observations, we present a novel approach inspired by the
conditional luminosity function method. We calibrate our model to observations
at z=6 and assume a non-evolving galaxy luminosity versus halo mass relation.
We first compare model predictions against the luminosity function measured at
z=5 and z=4. We then predict the luminosity function at z>=7 under the sole
assumption of evolution in the underlying dark-matter halo mass function. Our
model is consistent with the observed z>6.5 galaxy number counts in the HUDF
survey and suggests a possible steepening of the faint-end slope of the
luminosity function: alpha(z>8)< -1.9 compared to alpha=-1.74 at z=6. Although
we currently see only the brightest galaxies, a hidden population of lower
luminosity objects (L/L_{*}> 10^{-4}) might provide >75% of the total
reionizing flux. Assuming escape fraction f_{esc}~0.2, clumping factor C~5, top
heavy-IMF and low metallicity, galaxies below the detection limit produce
complete reionization at z>8. For solar metallicity and normal stellar IMF,
reionization finishes at z>6, but a smaller C/f_{esc} is required for an
optical depth consistent with the WMAP measurement. Our model highlights that
the star formation rate in sub-L_* galaxies has a quasi-linear relation to
dark-matter halo mass, suggesting that radiative and mechanical feedback were
less effective at z>6 than today.
