4.4 Lyman-alpha forest 

The Lyman-alpha 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. Although stringent observational constraints have been placed on competing cosmological models at large scales by the COBE satellite and over the smaller scales of our local Universe by observations of galaxies and clusters, there remains sufficient flexibility in the cosmological parameters that no single model has been established conclusively. 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.

Several combined N-body and hydrodynamic numerical simulations of the Lyman forest have been performed recently ([61, 94, 129], for example), and all have been able to fit the observations remarkably well, including the column density and Doppler width distributions, the size of absorbers [56], 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 ( , where is the baryonic density parameter, h is the Hubble parameter and is the photoionization rate at the hydrogen Lyman edge). However, see [45] for a discussion of the sensitivity of statistical properties on numerical resolution, and [93] for a systematic comparison of five different cosmological models to determine which attributes are sensitive physical probes or discriminators of models. A theoretical paradigm has thus emerged from these calculations in which Lyman-alpha absorption lines originate from the relatively small 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 6), including fluctuations in underdense regions, spheroidal minihalos, and filaments extending over scales of a few megaparsecs.

  

Figure 6: Distribution of the gas density at redshift z=3 from a numerical hydrodynamics simulation of the Lyman-alpha 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 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 =  K, light blue =  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.

Computational Cosmology: from the Early Universe to the Large Scale Structure Peter Anninos http://www.livingreviews.org/lrr-2001-2 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de