Comparison between the estimated HI metagalactic photoionisation rate
&Gamma-12, in units of 10-12s-1, from
QSO sources and the rate required by simulations to match the measured mean
Ly&alpha flux. The solid and dashed lines are estimates for the contribution
from QSOs by Haardt & Madau (1996), for two different QSO spectral shapes.
(From Meiksin & White 2004.)