An improved model for the effect of correlated Si  <scp>iii</scp> 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 Ly$\alpha$ – 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 Ly$\alpha$ and Si iii line profiles, and a variable ratio for coeval Ly$\alpha$ and Si iii optical depths. In contrast to earlier work, we also predict amplitudes for the Si iii power spectrum and Ly$\alpha$ – Si iii cross power spectrum that decrease towards lower redshift due to the hardening metagalactic UV background spectrum at $z \lesssim 3.5$. The fitting function is validated by comparison against multiple simulated data sets at redshifts $2.2 \leq z \leq 5.0$ and wavenumbers $k &amp;lt; 0.2$ s km$^{-1}$. Our model has little effect on existing warm dark matter constraints from the Ly$\alpha$ forest when adopting a physically motivated prior on the silicon abundance. It will, however, be an essential consideration for future, high precision Ly$\alpha$ forest power spectrum measurements.