Constraints on Quasar Lifetimes and Beaming from the HeII Lyman-alpha Forest

TL;DR

This study uses HeII Lyman-alpha forest observations to constrain quasar lifetimes and emission geometries, ruling out isotropic emission and infinite lifetimes, and suggesting moderate beaming or lifetimes of 10^7-10^8 years.

Contribution

It introduces a method to differentiate quasar emission models by analyzing radiation peaks and their displacements relative to quasars.

Findings

Isotropic emission and infinite lifetimes are ruled out at high confidence.

Moderate beaming or lifetimes of 10^7-10^8 years are consistent with observations.

Distribution of peak-quasar displacements can distinguish emission models.

Abstract

We show that comparisons of HeII Lyman-alpha forest lines of sight to nearby quasar populations can strongly constrain the lifetimes and emission geometry of quasars. By comparing the HeII and HI Lyman-alpha forests along a particular line of sight, one can trace fluctuations in the hardness of the radiation field (which are driven by fluctuations in the HeII ionization rate). Because this high-energy background is highly variable - thanks to the rarity of the bright quasars that dominate it and the relatively short attenuation lengths of these photons - it is straightforward to associate features in the radiation field with their source quasars. Here we quantify how finite lifetimes and beamed emission geometries affect these expectations. Finite lifetimes induce a time delay that displaces the observed radiation peak relative to the quasar. For beamed emission, geometry dictates that sources invisible to the observer can still create a peak in the radiation field. We show that both these models produce substantial populations of "bare" peaks (without an associated quasar) for reasonable parameter values (lifetimes ~10^6-10^8 yr and beaming angles <90 degrees). A comparison to existing quasar surveys along two HeII Lyman-alpha forest lines of sight rules out isotropic emission and infinite lifetime at high confidence; they can be accommodated either by moderate beaming or lifetimes ~10^7-10^8 yr. We also show that the distribution of radial displacements between peaks and their quasars can unambiguously distinguish these two models, although larger statistical samples are needed.