Carbon ionization states and the cosmic far-UV background with HeII absorption

TL;DR

This paper constrains the cosmic ultraviolet background spectrum at redshifts 2-3 by analyzing carbon ion ratios, revealing that HeII reionization was likely inhomogeneous, contrary to delayed reionization models.

Contribution

It introduces a method to constrain the UV background spectrum using carbon ion ratios and accounts for HeII absorption effects, providing new insights into HeII reionization.

Findings

Delayed reionization models are not supported by data.

HeII reionization was likely inhomogeneous.

The analysis favors models with less HeII at z~3.

Abstract

We constrain the spectrum of the cosmic ultraviolet background radiation by fitting the observed abundance ratios carbon ions at $z\sim 2\hbox{--}3$ with those expected from different models of the background radiation. We use the recently calculated modulation of the background radiation between 3 and 4 Ryd due to resonant line absorption by intergalactic HeII, and determine the ratios of CIII to CIV expected at these redshifts, as functions of metallicity, gas density and temperature. Our analysis of the observed ratios shows that 'delayed reionization' models, which assume a large fraction of HeII at $z\sim3$, is not favored by data. Our results suggest that HeII reionization was inhomogeneous, consistent with the predictions from recent simulations.