THE EVOLUTION OF THE DIFFUSE COSMIC ULTRAVIOLET BACKGROUND CONSTRAINED BY HUBBLE SPACE TELESCOPE OBSERVATIONS OF 3C 273

Recent Hubble Space Telescope UV spectroscopy of 3C 273 has revealed more low-redshift Lyman-alpha absorption lines (IGM clouds) than expected from the extrapolation from high-redshift (z greater-than-or-equal-to 1.6) observations. On the basis of the standard pressure confined cloud model of the Lyman-alpha forest, we show that this result indicates a sharp drop in the diffuse cosmic UV background from z = 2 to z = 0. If we fit a simple power law in 1 + z, the UV background flux must evolve as (1 + z)2.7 +/- 1. Applying this conclusion to a simple model for a photoionized diffuse component of the IGM, we predict that the H I optical depth (Gunn-Peterson effect) will also only drop slowly or perhaps even increase with decreasing redshift at z < 2. The implied constraints on the density and pressure of the diffuse IGM at z = 0 are also derived. We note that the inferred evolution of the diffuse UV flux bears a striking, and perhaps not coincidental, resemblance to the most recent direct determinations of the volume emissivity of the quasar population. Finally, we point out that information on the evolution of the UV flux carries interesting consequences for other cosmological issues such as galaxy formation; in particular, it seems to mitigate somewhat against relatively recent galaxy formation.