Quasar ionization of Lyman-alpha clouds - The proximity effect, a probe of the ultraviolet background at high redshift

Abstract

The authors study the distribution of lines in the Lyalpha forests in quasar spectra, using spectral data from 19 quasars with emission-line redshifts 1.7 < zQ < 3.8. The number density of Lyalpha lines generally increases with redshift z, but there exists a countervailing trend of dimishing number density within individual quasar spectra as z-->zQ.The authors give evidence that this countervailing trend is due to enhanced ionization of Lyalpha clouds by the bright, nearby quasars in whose spectra they are observed (the "proximity effect"). The authors develop a quantitative physical model for this effect, and using a variety of statistical tests, they show that the model fits the data; furthermore, they infer the background of ionizing photons at large z by optimizing the fit. Assuming the model is correct, this represents the first physical measurement of the high-z UV background.