He II absorption and the sawtooth spectrum of the cosmic far-UV background

TL;DR

This paper investigates how resonant line absorption in intergalactic He II causes a sawtooth pattern in the cosmic far-UV background spectrum, providing insights into helium reionization and IGM ionization.

Contribution

It introduces the concept of sawtooth modulation in the UV background spectrum caused by He II absorption, linking it to helium reionization and IGM ionization modeling.

Findings

Sawtooth modulation pattern depends on He II absorption opacity.

Spectrum peaks just above each resonance frequency.

Modulation affects metal ion abundance modeling in the IGM.

Abstract

Cosmic ultraviolet background radiation between 3 and 4 Ryd is reprocessed by resonant line absorption in the Lyman series of intergalactic He II. This process results in a sawtooth modulation of the radiation spectrum from the He II Lya frequency to the Lyman limit. The size of this modulation is a sensitive probe of the epoch of helium reionization and of the sources that keep the intergalactic medium (IGM) highly ionized. For large absorption opacities, the background intensity will peak at frequencies just above each resonance, go to zero at resonance, and fluctuate greatly just below resonance. The He II sawtooth modulation may be one of the missing ingredients needed in the modelling of the abundances of metal ions like C III and Si IV observed in the IGM at redshift 3.