Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the ΛCDM Universe

This study analyzes the sizes, shapes, and correlations of Lyα clouds produced by a hydrodynamic simulation of a spatially flat CDM universe with a nonzero cosmological constant (Ω 0 = 0.4, Λ 0 = 0.6, σ 8 = 0.79) over the redshift range 2 ≤ z ≤ 4. The Lyα clouds range in size from several kiloparsecs to about a hundred kiloparsecs in proper units, and they range in shape from roundish, high column density regions with N H I ≥ 10 15 cm -2 to low column density sheet-like structures with N H I ≤ 10 13 cm -2 at z = 3. The most common shape found in the simulation resembles that of a flattened cigar. The physical size of a typical cloud grows with time roughly as (1 + z ) -3/2 , while its shape hardly evolves (except for the most dense regions with ρ cut > 30). Collectively, the clouds form large networks of filaments and sheets. Our result indicates that any simple model with a population of spheres (or other shapes) of a uniform size is oversimplified; if such a model agrees with observational evidence, it is probably only by coincidence. We also illustrate why the use of pairs of quasar sight lines to set lower limits on cloud sizes is useful only when the perpendicular sight line separation is small (Δ r ≤ 50 h -1 kpc). Finally, we conjecture that high column density Lyα clouds ( N H I ≥ 10 15 cm -2 ) may be the progenitors of the lower redshift faint blue galaxies, based on consideration of their correlation, number density, and mass.