Separating the Memory of Reionisation from Cosmology in the Lyα Forest Power Spectrum at the Post-Reionisation Era

It has been recently shown that the astrophysics of reionisation can be extracted from the Ly alpha forest power spectrum by marginalizing the memory of reionisation over cosmological information. This impact of cosmic reionisation on the Ly alpha forest power spectrum can survive cosmological time-scales because cosmic reionisation, which is inhomogeneous, and subsequent shocks from denser regions can heat the gas in low-density regions to similar to 3 x 10(4) K and compress it to mean-density. Current approach of marginalization over the memory of reionisation, however, is not only model-dependent, based on the assumption of a specific reionisation model, but also computationally expensive. Here, we propose a simple analytical template for the impact of cosmic reionisation, thereby treating it as a broad-band systematic to be marginalized over for Bayesian inference of cosmological information from the Ly alpha forest in a model-independent manner. This template performs remarkably well with an error of <= 6 per cent <= 6 per cent at large scales k asymptotic to 0.19 Mpc(-1), where the effect of the memory of reionisation is important, and reproduces the broad-band effect of the memory of reionisation in the Ly alpha forest correlation function, as well as the expected bias of cosmological parameters due to this systematic. The template can successfully recover the morphology of forecast errors in cosmological parameter space as expected when assuming a specific reionisation model for marginalization purposes, with a slight overestimation of tens of per cent for the forecast errors on the cosmological parameters. We further propose a similar template for this systematic on the Ly alpha forest 1D power spectrum.