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Abstract

We present an analysis of Si-III absorption and its effect on the 1D Ly$\alpha$ forest power spectrum using the Sherwood-Relics hydrodynamical simulation suite. In addition to the well-understood oscillations arising from the Ly$\alpha$--Si-III cross correlation, 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 toward 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 datasets at redshifts $2.2\leq z \leq 5.0$ and wavenumbers $k < 0.2\rm\,s\,km^{-1}$. Our model remains in good agreement with earlier work at large scales ($k \lesssim 0.06\rm\,s\,km^{-1}$) and it 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.