An improved model for the effect of correlated Si III absorption on the one-dimensional Lyman-α forest power spectrum

Abstract

We present an analysis of Si III absorption and its effect on the one-dimensional Ly forest power spectrum using the Sherwood–Relics hydrodynamical simulation suite. In addition to oscillations from the Lyα-Si III cross-correlation that are damped towards smaller scales, we find an enhancement in small-scale power that has been ignored in previous studies. We therefore develop a new analytical fitting function that captures two critical effects that have previously been neglected: distinct Lyα and Si III line profiles, and a variable ratio for coeval Lyα and Si III optical depths. In contrast to earlier work, we also predict amplitudes for the Si III power spectrum and Lyα-Si III cross power spectrum that decrease towards lower redshift due to the hardening metagalactic UV background spectrum at z ≲ 3.5. The fitting function is validated by comparison against multiple simulated data sets at redshifts 2.2 ≤ z ≤ 5.0 and wavenumbers k < 0.2 s km-1. Our model has little effect on existing warm dark matter constraints from the Lyα forest when adopting a physically motivated prior on the silicon abundance. It will, however, be an essential consideration for future, high precision Lyα forest power spectrum measurements. © The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.