3.6 Galaxy Clusters3 Physical Cosmology3.4 Gravitational Lensing

3.5 Ly tex2html_wrap_inline810 Forest 

The Ly tex2html_wrap_inline810 forest represents the optically thin (at the Lyman edge) component of Quasar Absorption Systems (QAS), a collection of absorption features in QSO 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 Ly tex2html_wrap_inline810 forest have been performed recently [28, 47, 64], and all have been able to fit the observations remarkably well, including the column density and Doppler width distributions, the size of absorbers [26], and the line number evolution. Despite the fact that the cosmological models and parameters are different in each case, the simulations give similar results provided that the proper ionization bias is used (tex2html_wrap_inline1004, where tex2html_wrap_inline1006 is the baryonic density parameter, h is the Hubble parameter and tex2html_wrap_inline1010 is the photoionization rate at the hydrogen Lyman edge). A theoretical paradigm has thus emerged from these calculations in which Ly tex2html_wrap_inline810 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, including fluctuations in underdense regions, spheroidal minihalos, and filaments extending over scales of a few megaparsecs (figure 4). However, it is not yet clear what effect different cosmological models have on these systems.

  

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Figure 4: Distribution of the gas density at redshift z =3 from a numerical hydrodynamics simulation of the Ly tex2html_wrap_inline810 forest. The simulation adopted a CDM spectrum of primordial density fluctuations, normalized to the second year COBE observations, a Hubble parameter of h =0.5, a comoving box size of 9.6 Mpc, and baryonic density of tex2html_wrap_inline818 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 cosmic density (characteristic of typical filamentary structures) and are color coded to the gas temperature (dark blue = tex2html_wrap_inline820 K, light blue = tex2html_wrap_inline822 K). The higher density contours trac e 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 tex2html_wrap_inline824 mass fraction is shown with a wire mesh in this same slice. Notice that there is fine structure everywhere. 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.

3.6 Galaxy Clusters3 Physical Cosmology3.4 Gravitational Lensing

image Physical and Relativistic Numerical Cosmology
Peter Anninos
http://www.livingreviews.org/lrr-1998-2
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