The low-redshift Ly alpha forest in cold dark matter cosmologies

We study the physical origin of the low-redshift Ly alpha forest in hydrody namic simulations of four cosmological models, all variants of the cold dar k matter scenario. Our most important conclusions are insensitive to the co smological model, but they depend on our assumption that the UV background declines at low redshift in concert with the declining population of quasar sources. We fmd that the expansion of the universe drives rapid evolution of dN/dz (the number of absorbers per unit redshift above a specified equiv alent width threshold) at z greater than or similar to 1.7, but that at low er redshift the fading of the UV background counters the influence of expan sion, leading to slow evolution of dN/dz. The draining of gas from low-dens ity regions into collapsed structures has a mild but not negligible effect on the evolution of dN/dz, especially for high equivalent-width thresholds. At every redshift, weaker lines come primarily from moderate fluctuations of the diffuse, unshocked intergalactic medium (IGM) and stronger lines ori ginate in shocked or radiatively cooled gas of higher overdensity. However, the neutral hydrogen column density associated with structures of fixed ov erdensity drops as the universe expands, so an absorber at z = 0 is dynamic ally analogous to an absorber that has column density 10-50 times higher at z = 2-3. In particular, the mildly overdense IGM fluctuations that dominat e the Ly alpha forest opacity at z > 2 produce optically thin lines at z < 1, while the marginally saturated (N-HI similar to 10(14.5) cm(-2)) lines a t z < 1 typically arise in gas that is overdense by a factor of 20-100. We find no dear distinction between lines arising in "galaxy halos" and lines arising in larger scale structures; however, galaxies tend to lie near the dense regions of the IGM that are responsible for strong Ly alpha lines. Th e simulations provide a unified physical picture that accounts for the most distinctive observed properties of the low-redshift Ly alpha forest: (1) a sharp transition in the evolution of dN/dz at z similar to 1.7, (2) strong er evolution for absorbers of higher equivalent width, (3) a correlation of increasing Ly alpha equivalent width with decreasing galaxy impact paramet er that extends to r(p) similar to 500 h(-1) kpc, and (4) a tendency for st ronger lines to arise in close proximity to galaxies while weaker lines tra ce more diffuse large-scale structure.