Physical Properties of the Lyα Forest in a Cold Dark Matter Cosmology

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Abstract
We discuss the origin and physical nature of the Lyα forest absorption systems as found in hydrodynamical simulations of the intergalactic medium (IGM) in a standard cold dark matter cosmology (Ω = 1, H0 = 50 km s-1 Mpc-1, σ8 = 0.7). The structures of the systems that give rise to the Lyα forest span a wide range in morphologies, depending on the density contrast. Highly overdense systems, ρb/b10, where ρb denotes baryon density, tend to be spheroidal and are located at the intersections of an interconnecting network of filaments of moderate overdensity, 1ρb/b5. The typical thickness of the filaments is 100 kpc, with a typical length of a few megaparsecs. At the cosmological average density, the characteristic morphology is cell-like with underdense regions separated by overdense, sheetlike partitions. The lowest density contours tend to enclose amorphous, isolated regions. We find that the principal structures of the IGM are in place by z = 5, with the evolution in the IGM absorption properties due primarily to the expansion of the universe and the changing intensity of the photoionizing background radiation field. The absorption properties of the forest clouds correlate strongly with those of the underlying physical systems from which they arise. The highest column density systems (log NH I 15) correspond to the highly overdense spheroidal structures, moderate column density systems (13 log NH I 14) correspond to the filaments, and the lowest density absorption systems originate from discrete fluctuations within underdense regions a few megaparsecs across, cosmic minivoids. Most of the intergalactic He II opacity arises from these underdense regions. Similar correlations are found for the cloud temperature and divergence of the peculiar velocity field. Within the uncertainties of the statistics of the derived Lyα forest properties, we are able to account for the distribution of optical depths in our synthesized spectra entirely by absorption due to discrete systems. We find that virtually all the baryons in the simulation fragment into structures that we can identify with discrete absorption lines, with at most 5% remaining in a smoothly distributed component (the Gunn-Peterson effect). We compare our results with the cloud ionization parameters inferred from Keck HIRES measurements of carbon and silicon in the Lyα forest. Combining with constraints imposed by measurements of the mean intergalactic H I opacity permits separate limits to be set on the mean cosmological baryon density, Ωb, and H I ionization rate, ΓH I. For the cosmological model investigated, we find 0.03 Ωb 0.08 and 0.3 ΓH I,-12 1 (ΓH I,-12 = ΓH I/10-12 s-1) at z = 3-3.5. Our results for the amount of intergalactic H I and He II absorption and for the ionization parameters of the clouds are consistent with a forest photoionized by a UV background dominated by QSO sources with an intrinsic spectral index of αQ ≈ 1.8-2.
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