Cumulative ratio of the number of produced (thin lines) and escaped (thick lines) ionizing photon to the average hydrogen density. The solid blue, dashed orange and dotted green lines correspond to the total contribution and that of stellar populations and AGN, respectively. For better readability, we show the intrinsic stellar contribution with dots instead of a line. The background shaded area indicates the z₀₁ − z₉₉ (z₁₀ − z₉₀) redshift intervals. The grey area highlights a photon-to-baryon ratio between 1 and 3, necessary to reionize the Universe. Stellar populations alone provide the necessary number of photons by z ∼ 6.