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The next component to be considered is the all-pervading intergalactic medium which manifests itself as a multitude of individual Lyalpha absorption lines bluewards of the Lyalpha emission line of every QSO. As can be appreciated from Figures 13 and 14, the effect is dramatic at high redshift. Observationally, the term Lyalpha forest is used to indicate absorption lines with column densities in the range 1016 gtapprox N(HI) gtapprox 1012 cm-2 (see Figure 5).

Figure 13

Figure 13. (Reproduced from Ellison 2000). This is one of the best QSO spectra ever obtained thanks to the combination of the bright magnitude of the gravitationally lensed QSO Q1422+231 (V = 16.5), long exposure time (amounting to several nights of observation), and high spectral resolution offered by the Keck echelle spectrograph (FWHM appeq 8 km s-1). The signal-to-noise ratio in the continuum longwards of the Lyalpha emission line is between 200 and 300. At these high redshifts (zem = 3.625) the Lyalpha forest eats very significantly into the QSO spectrum below the Lyalpha emission line and, with the present resolution, breaks into hundreds of absorption components (see Figure 14).

Figure 14

Figure 14. (Reproduced from Ellison 2000). Portion of the Lyalpha forest between zabs = 3.277 and 3.607 in the Keck spectrum of Q1422+231 shown in Figure 13. There are more than 50 individual absorption components in each 100Å-wide stretch of spectrum.

Hydrodynamical simulations have shown that the Lyalpha forest is a natural consequence of the formation of large scale structure in a universe dominated by cold dark matter and bathed in a diffuse ionising background (see Weinberg, Katz, & Hernquist 1998 for an excellent review of the ideas which have led to this interpretation). An example is reproduced in Figure 15. Artificial spectra generated by throwing random sightlines through such model representations of the high redshift universe are a remarkably good match to real spectra of the Lyalpha forest. In particular, the simulations are very successful at reproducing the column density distribution of H I in Figure 5, the line widths and profiles, and the evolution of the line density with redshift. Consequently, much of what we have learnt about the IGM in the last few years has been the result of a very productive interplay between observations of increasing precision and simulations of increasing sophistication. This modern view of the Lyalpha forest is often referred to as the `fluctuating Gunn-Peterson' effect.

There are two important properties of the Lyalpha forest which we should keep in mind. One is that it is highly ionised, so that the H I we see directly is only a small fraction (~ 10-3 to ~ 10-6) of the total amount of hydrogen present. With this large ionisation correction it appears that the forest can account for most of the baryons at high, as well as low, redshift (Rauch 1998; Penton, Shull, & Stocke 2000); that is OmegaLyalpha approx 0.02 h-2. Second, the physics of the absorbing gas is relatively simple and the run of optical depth tau(Lyalpha) with redshift can be thought of as a `map' of the density structure of the IGM along a given line of sight. At low densities, where the temperature of the gas is determined by the balance between photoionisation heating (produced by the intergalactic ionising background) and adiabatic cooling (due to the expansion of the universe), tau(Lyalpha) propto (1 + delta)1.5, where delta is the overdensity of baryons delta ident (rhob / <rhob> - 1). At z = 3, tau(Lyalpha) = 1 corresponds to a region of the IGM which is just above the average density of the universe at that time (delta approx 0.6). The last absorption line in the second panel of Figure 14, near 5395Å, is an example of a Lyalpha line with tau appeq 1. The idea that, unlike galaxies, the forest is an unbiased tracer of mass has prompted, among other things, attempts to recover the initial spectrum of density fluctuations from consideration of the spectrum of line optical depths in the forest (Croft et al. 2002)

Figure 15

Figure 15. (Reproduced from Distribution of neutral gas at z = 3 from hydrodynamic cosmological simulation in a spatially flat, COBE-normalized, cold dark matter model with the cosmological parameters adopted in this article (Section 1.1). The box size is 25 Mpc / h (comoving) on the side, and the number of particles used in the simualation is 7683. The structure seen in this (and other similar simulations) reproduces very well the spectral properties of the Lyalpha forest when artificial spectra are generated along random sightlines through the box.

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