The Soft, Fluctuating UVB at $z\sim6$ as Traced by C IV, SiIV, and CII

TL;DR

This study compares different models of the ultraviolet background at redshift 6 using metal absorbers, finding that a soft, fluctuating UVB best matches observations, supporting a Population II star-driven reionization scenario.

Contribution

It provides the first comparison of observed metal absorber distributions with simulations using various UVB models at z~6, highlighting the preference for a soft, fluctuating UVB.

Findings

A soft, fluctuating UVB best reproduces observed metal ion ratios.

QSO-only UVB overpredicts high-ionization absorbers, indicating it is too hard.

Current data favor a Population II star-driven UVB over a QSO-dominated one.

Abstract

The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): Bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller-scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at $z\sim6$ versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially-inhomogeneous "galaxies+QSOs" UVB; a homogeneous "galaxies+QSOs" UVB (Haardt & Madau 2012); and a QSOs-only model. All UVBs reproduce the observed column density distributions of CII, SiIV, and CIV reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed SiIV/CIV and CII/CIV distributions. The QSOs-only UVB overpredicts both CIV/CII and CIV/SiIV, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts CIV/SiIV, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of two deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.