Redshift evolution of the Lyman-line-absorbing clouds in quasar spectra

The spectral resolution of the results of the present analysis of absorption lines from an echelle spectrum of the z = 3.12 quasar Q0420-388 is sufficiently high to resolve most of the Lyman absorption lines, allowing velocity dispersions and column densities to be obtained by profile fitting. By comparing the properties of the Lyman absorption systems at redshifts of about 2 with those at about 3, it is noted that there is no evidence for redshift evolution in the distribution function of any measured quantity. Simple models in which a hot intergalactic medium pressure confined a comoving population of clouds are consistent with the available data, if the clouds are nearly in thermal and ionization balance with the ionizing background flux from quasars.