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1.1. Intergalactic Hydrogen Density

The proper mean density of hydrogen nuclei at redshift z may be expressed in standard cosmological terms as:

Equation 1 (1)
           Equation 2 (2)

where Y is the primordial He abundance by mass, rhocrit = 3H02 / (8 pi G) is the critical density, Omegab = rhob / rhocrit is the current baryonic density parameter, and H0 = 100 h km s-1 Mpc-1 is the present-day Hubble constant. Standard nucleosynthesis models together with recent observations of Deuterium yield Y = 0.247 ± 0.02, and Omegab h2 = 0.0193 ± 0.0014. Thus,

Equation 3 (3)

As some of the baryons had already collapsed into galaxies at z = 2-5, the value of Omegab h2 = 0.019 should strictly be considered as an upper limit to the intergalactic density parameter.

Because of the overwhelming abundance of hydrogen, the ionization of this element is of great importance for determining the physical state of the IGM. Popular cosmological models predict that most of the intergalactic hydrogen was reionized by the first generation of stars or quasars at z = 7-15. The case that has received the most theoretical studies is one where hydrogen is ionized by the absorption of photons, H + gamma -> p + e (as opposite to collisional ionization H + e -> p + e + e) shortward of 912 Å; that is, with energies exceeding 13.6 eV, the energy of the Lyman edge. The process of reionization began as individual sources started to generate expanding H II regions in the surrounding IGM; throughout an H II region, H is ionized and He is either singly or doubly ionized. As more and more sources of ultraviolet radiation switched on, the ionized volume grew in size. The reionization ended when the cosmological H II regions overlapped and filled the intergalactic space.

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