EVOLUTION OF THE LYMAN-ALPHA FOREST FROM HIGH TO LOW-REDSHIFT

We study the evolution with redshift, from z similar to 5 to z = 0, of the Lyman alpha forest in a CDM model using numerical simulations inc luding collisionless particles only. The baryonic component is assumed to follow the dark matter distribution. We distinguish between two po pulations of particles: Population P-s traces the filamentary structur es of the dark matter, evolves slowly with redshift and, for N(HI) gre ater than or similar to 10(14) cm(-2), dominates the number density of lines at z < 3; most of population P-u is located in underdense regio ns and for the same column densities, disappears rapidly at high redsh ift. We generate synthetic spectra from the simulation and show that t he redshift evolution of the Lyman alpha forest (decrement, N(HI) dist ribution) is well reproduced over the whole redshift range for Omega(b )h(2) similar to 0.0125 and J(-21) approximate to 0.1 at z similar to 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) greater than or similar to 10(12) cm(-2) remains approximately constan t from z similar to 4 to z = 1. At z similar to 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 (w(r) > 0.3 Angstrom) lines are expected to be associat ed with galaxies, the bulk of the Lyman alpha forest however should ha ve lower equivalent width and should not be tightly correlated with ga laxies.