What Keeps the Universe Ionized at z=5?
  The history of the transition from a neutral intergalactic medium to one that is almost fully ionized can reveal the character of cosmological ionizing sources. In this talk I will discuss the implications for rival reionization scenarios of the rapid decline observed in the space density of quasars and star-forming galaxies at redshifts z>3. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of ionizing photons emitted in one recombination time equals the mean number of hydrogen atoms. At z=5, the local character of the UV metagalactic flux allows one to define a critical emission rate of hydrogen-ionizing photons per unit comoving volume. Models based on photoionization by bright QSOs and/or young galaxies with star formation rates in excess of 0.3-1 M_sun/yr appear to fail to provide the required number of hydrogen-ionizing photons at these redshifts by large factors. If stellar sources are responsible for keeping the universe ionized at z=5, the rate of star formation per unit comoving volume at this epoch must be comparable or greater than observed at z=3. 
Piero Madau (STScI)