Evolution of the Lyman-alpha forest from high to low redshift

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We study the evolution with redshift, from z=5 to z=0, of the Lyman-alpha forest in a CDM model using numerical simulations including collisionless particles only. The baryonic component is assumed to follow the dark matter distribution. We distinguish between two populations of particles: Population Ps traces the filamentary structures of the dark matter, evolves slowly with redshift and, for N(HI)>10¹4 cm-2, dominates the number density of lines at z<3; most of population Pu is located in underdense regions and for the same column densities, disappears rapidly at high redshift. We generate synthetic spectra from the simulation and show that the redshift evolution of the Lyman-alpha forest (decrement, N(HI) distribution) is well reproduced over the whole redshift range for Omegab*h²=0.0125 and J-21=0.1 at z=3 where J-21 is the UV background flux intensity in units of 10-21 erg cm-2 s-1 Hz-1 sr-1. The total number of lines with N(HI)>10¹2 cm-2 remains approximately constant from z=4 to z=1. At z=0, the number density of lines per unit redshift with log N(HI)>12, 13, 14 is of the order of 400, 100, and 20 respectively. Therefore, at low redshift, if most of the strong (wᵣ>0.3 Å) lines are expected to be associated with galaxies, the bulk of the Lyman-alpha forest however should have lower equivalent width and should not be tightly correlated with galaxies.