The redshift evolution of Lyman-alpha absorbers

We present a model for the Lyman-alpha absorbers that treats all objects (f rom the low-density forest clouds to the dense damped systems) in a unified description. This approach is consistent with an earlier model of galaxies (luminosity function, metallicity) but also with the known description of the density field in the small-scale non-linear regime. We consider two cos mological models: a critical universe Omega = 1 with a CDM power-spectrum, and an open CDM universe with Omega(0) = 0.3, Lambda = 0. we reproduce the available data on column density distribution as a function of redshift, th e value of the main new parameter, the background ionizing UV flux, being c onsistent with the observed limits. This allows a quantitatively trustable analytical description of the opacity, mass, size, velocity dispersion and metallicity of these absorbers, over a range of column densities spanning 1 0 orders of magnitude. Moreover, together with an earlier model of galaxy f ormation this draws a unified picture of the redshift evolution of structur es in the universe, from underdense clouds to massive high density galaxies , from weak to very deep potential wells.