The influence of metagalactic ultra-violet background fluctuations on the high-redshift Ly$\alpha$ forest

TL;DR

This paper investigates how fluctuations in the ultra-violet background, mainly caused by QSOs, influence the Lyα forest at high redshifts, showing that these fluctuations can explain observed optical depth variations.

Contribution

It demonstrates that QSO shot noise dominates UV background fluctuations at high redshifts and that these fluctuations can account for the broad optical depth distribution in the Lyα forest.

Findings

QSO shot noise causes UV background fluctuations with large correlation lengths.

Fluctuations can explain the broad distribution of effective optical depths.

Large simulation boxes are needed to capture these fluctuations accurately.

Abstract

Under the assumption that galaxies and Quasi-Stellar Objects (QSOs) dominate the metagalactic ultra-violet (UV) background, it is shown that at high redshifts fluctuations in the UV background are dominated by QSO shot noise and have an auto-correlation length of a few to several comoving Mpcs, depending on the bright end of the QSO luminosity function. The correlations create long range spatial coherence in the neutral hydrogen fraction. Using a semi-analytic model, it is demonstrated that the coherence may account for the broad distribution in effective optical depths measured in the Ly$\alpha$ forest spectra of background QSOs, for line-of-sight segments of comoving length $50h^{-1}$ Mpc at redshifts $5<z<6$. Capturing the fluctuations in a numerical simulation requires a comoving box size of $\sim1h^{-1}$ Gpc, although a box half this size may be adequate if sufficient random realizations of the QSO population are performed.