Fraction of the critical specific UV luminosity density necessary for sustaining reionization (denoted by the gray bar for C/ f _esc = 10) obtained from the (incompleteness-corrected) observed galaxy population (large cyan points) as a function of redshift, assuming C/ f _esc = 10 (left vertical axis) and 30 (right vertical axis). The blue inverted triangles, green squares, and red triangles show the critical fraction for galaxies with luminosities greater than 0.2, 0.5, and 1 × the characteristic luminosity L*, respectively. At z = 6, where the observed population can reionize the universe, this figure highlights that while galaxies below our detection limit are not necessary, it is the fainter portion of the observed population which is dominating ρ _UV, as galaxies with luminosities greater than 0.2 L* contribute ~2.7× more than L > L* galaxies. This is even more so in the case at z = 7 and 8, though at these redshifts the entire observed population is short by a factor of ~2.5–3 of the ionizing photons necessary to sustain reionization for C/ f _esc = 10.