STScI Preprint #1237

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, N^._ion = 10^51.3±0.3 phot s^-1 Mpc^-3. Models based on photoionization by bright QSOs and/or young galaxies with star formation rates in excess of 0.3-1M yr^-1 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.

Status:

Appeared in: Mem. Soc. Astron. Ital. 69: 471-479, 1998
(the proceedings of Molecular Hydrogen in the Early Universe, held December 1997 in Florence, Italy)

Affiliations:

Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218, USA