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Дата изменения: Wed May 24 00:02:43 2000 Дата индексирования: Sat Dec 22 15:28:27 2007 Кодировка: Поисковые слова: ionized hydrogen |
STScI Preprint #1237
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 = 1051.3±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.
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