4.4 Lyα forest

The Lyα forest represents the optically thin (at the Lyman edge) component of Quasar Absorption Systems (QAS), a collection of absorption features in quasar spectra extending back to high redshifts. QAS are effective probes of the matter distribution and the physical state of the Universe at early epochs when structures such as galaxies are still forming and evolving. The relative lack of constraining observational data at the intermediate to high redshifts (0 < z < 5), where differences between competing cosmological models are more pronounced, suggests that QAS can potentially yield valuable and discriminating observational data.

Many complex multi-component numerical simulations have been performed of the Lyman forest, which include the effects of dark matter (N-body), baryons (hydrodynamics), chemical composition (reactive networks), and microphysical response (radiative cooling and heating). See, for example, [67118166], which represent some of the earliest comprehensive simulations. For the most part, all these calculations have been able to fit the observations reasonably well, including the column density and Doppler width distributions, the size of absorbers [62], and the line number evolution. Despite the fact that the cosmological models and parameters are different in each case, the simulations give roughly similar results provided that the proper ionization bias is used, bion ≡ (Ωbh2)2∕Γ, where Ωb is the baryonic density parameter, h is the Hubble parameter and Γ is the photoionization rate at the hydrogen Lyman edge. (However, see [50] for a discussion of the sensitivity of statistical properties on numerical resolution.) A theoretical paradigm has thus emerged from these calculations in which Lyα absorption lines originate from the relatively smaller scale structure in pregalactic or intergalactic gas through the bottom-up hierarchical formation picture in CDM-like Universes. The absorption features originate in structures exhibiting a variety of morphologies commonly found in numerical simulations (see Figure 11View Image), including fluctuations in underdense regions, spheroidal minihalos, and filaments extending over scales of a few Mpc.

View Image

Figure 11: Distribution of the gas density at redshift z = 3 from a numerical hydrodynamics simulation of the Lyα forest with a CDM spectrum normalized to second year COBE observations, Hubble parameter of h = 0.5, a comoving box size of 9.6 Mpc, and baryonic density of Ωb = 0.06 composed of 76% hydrogen and 24% helium. The region shown is 2.4 Mpc (proper) on a side. The isosurfaces represent baryons at ten times the mean density and are color coded to the gas temperature (dark blue = 3 × 104 K, light blue = 3 × 105 K). The higher density contours trace out isolated spherical structures typically found at the intersections of the filaments. A single random slice through the cube is also shown, with the baryonic overdensity represented by a rainbow-like color map changing from black (minimum) to red (maximum). The He+ mass fraction is shown with a wire mesh in this same slice. To emphasize fine structure in the minivoids, the mass fraction in the overdense regions has been rescaled by the gas overdensity wherever it exceeds unity.
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Machacek et al. [115] expanded on earlier work to compare several Lyα statistical measures from five different background cosmological models, including standard critical density Cold Dark Matter (CDM), open CDM, flat CDM with a cosmological constant, standard CDM with a tilted density spectrum, and a flat model with mixed hot and cold dark matter. All models were chosen to match local or low redshift observations, and most were also consistent with COBE measurements of the CMBR. The calculations were designed to establish which statistics are sensitive to different cosmological models. In particular, they find that the line number count above a given column density threshold is relatively insensitive to background models. On the other hand, the shape of the optical depth probability distribution function is strongly correlated to the amount of small scale power in density fluctuations, and is thus a good discriminator among models on scales of a few hundred kpc.

Meiksin et al. [117] followed up with more detailed comparisons of Lyα systems in several cosmologies with observed high resolution QSO spectra. Although all models are consistent with previous studies in that they give reasonably good statistical agreement with observed Lyα properties, under closer scrutiny none of the numerical models they considered passed all the tests, which included spectral flux, wavelet decomposed amplitude, and absorption line profile distributions. They suggest that comparisons might be improved, particularly in optically thin systems, by more energy injection into the IGM from late He+ reionization or supernovae-driven winds, or by a larger baryon fraction.

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