Aligned metal absorbers and the ultraviolet background at the end of reionization

TL;DR

This study uses observations of aligned metal absorbers at high redshift to compare with simulations, revealing preferences for different ultraviolet background models and highlighting the potential of future observations to better constrain the UVB during reionization.

Contribution

It introduces a method of using aligned metal absorbers to distinguish between different models of the ultraviolet background at the end of reionization.

Findings

Observed C IV/Si IV absorbers favor a hard, uniform UVB.

Some ion pairs prefer the HM12 UVB model.

Future observations can tighten constraints on the UVB.

Abstract

We use observations of spatially-aligned C II, C IV, Si II, Si IV, and O I absorbers to probe the slope and intensity of the ultraviolet background (UVB) at $z \sim 6$. We accomplish this by comparing observations with predictions from a cosmological hydrodynamic simulation using three trial UVBs applied in post-processing: a spectrally soft, fluctuating UVB calculated using multi-frequency radiative transfer; a soft, spatially-uniform UVB; and a hard, spatially-uniform "quasars-only" model. When considering our paired high-ionization absorbers (C IV/Si IV), the observed statistics strongly prefer the hard, spatially-uniform UVB. This echoes recent findings that cosmological simulations generically underproduce strong C IV absorbers at $z>5$. A single low/high ionization pair (Si II/Si IV), by contrast, shows a preference for the HM12 UVB, while two more (C II/C IV and O I/C IV) show no preference for any of the three UVBs. Despite this, future observations of specific absorbers, particularly Si IV/C IV, with next-generation telescopes probing to lower column densities should yield tighter constraints on the UVB.