EVOLUTION OF THE IONIZING BACKGROUND AT HIGH REDSHIFTS

The decrease in number density of Lyman alpha clouds near the backgrou nd quasar is an observational result that is often called the 'proximi ty' or 'inverse' effect. It is thought that, for nearby clouds, the fl ux of the quasar dominates the background radiation held, increasing t he ionization state of the clouds and reducing the (observed) H I colu mn density. In this paper we analyse a sample of 11 quasars from the l iterature for which accurate column density estimates of the Lyman alp ha lines exist. We confirm, to a significance level of more than 3 sig ma, that the proximity effect exists. If it is related to the backgrou nd flux then the intensity and evolution of the background have been c onstrained. Using a maximum-likelihood method, we determine the streng th of the extragalactic ionizing background for 2.0 < z < 4.5, taking account of possible systematic errors in our determination and estimat ing the effect of biases inherent in the data. If the background is co nstant we find that it has an intensity of 100(-30)(+50)J(23), where J (23) is defined as 10(-23) erg s(-1) cm(-2) sr(-1) Hz(-1). There is no significant evidence for a change in this value with redshift.