The Proximity Effect on the Lyman α Forest Due to a Foreground QSO

Abstract

We present the results of our study on three close pairs of QSOs. Our results are consistent with the existence of a proximity effect. due to the foreground QSO, but due to its weakness we can only reject the absence of such effect at ~ 1σ level. By modeling this proximity effect in terms of a simple photoionisation model, we find the best value for the UV ionizing background to be ~ 10^−20.5 erg cm⁻² s⁻¹ Hz⁻¹ srad⁻¹ at the Lyman limit, and an absolute lower limit (95% confidence) of 10^−21.8 erg cm⁻² s⁻¹ Hz⁻¹ srad⁻¹. This lower limit rejects a number of models for the UV background where it is mostly contributed by QSOs and absorption by Lyman limit systems is taken into account.