Lyα forest Sample Clauses

Lyα forest. Discovered in 1970 [60], the Lyα forest is a powerful tool in cosmology to study the intergalactic medium (IGM). The Lyα forest appears as a series of absorption features (or “lines”) in the smooth thermal spectrum of dis- tant quasars (also called quasi-stellar objects or QSOs), active galactic nuclei present at redshifts up to z 5 6. Lyα photon absorption originates from the O(10) eV transition between states n = 1 ! n = 2 in hydrogen atoms, where n = 1 hydrogen is often referred to as “neutral” hydrogen or HI. As the quasar light travels to us through the IGM it crosses areas of hydrogen overdensities, which can be described by the hydrogen power spectrum, and which contains a fraction of neutral hydrogen. The average fraction of neutral hydrogen depends mainly on the physics during the epoch of reion- ization (EoR), which we will discuss again in the next chapter, and happens after non-linear structure growth of hydrogen begins (ie the formation of the first stars and galaxies). In turn, this causes the release of radiation which forms “bubbles” of excited and ionized hydrogen atoms around these first structures. The EoR is often believed to have occurred around z 10 20, after which time the neutral IGM is considered largely re-ionized. Therefore at z 5 6 the fraction of neutral hydrogen we observe through absorption of Lyα radiation is quite small (1 part in 104 [61]).6 We can also model the lo- cal fraction of neutral hydrogen more carefully. Under the assumption that the IGM is mainly composed of hydrogen, baryonic density perturbations D(x; z) can be related to the neutral hydrogen density [11, 62, 63]: nHI(x; z) = nˆHI(z)Dα (x; z) ; (2.27) where D = ρ b=hρ bi is the fractional baryonic density perturbation, α = (2:7 0:7)ζ and nˆHI is the neutral hydrogen density at mean baryonic den- sity distribution which depends in particular on the photoionization rate of the IGM [61, 15]. The power-law index ζ was measured with a range of different methods, see [64, 65, 66, 67, 68, 69, 70, 15], finding indices around ζ = 1:4 in the redshift range of interest.
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