The impact of temperature fluctuations on the large-scale clustering of the Ly$α$ forest

Abstract

We develop a semi-analytic method for assessing the impact of the large-scale IGM temperature fluctuations expected following He${\rm\,{\scriptstyle II}}$ reionization on three-dimensional clustering measurements of the Ly$α$ forest. Our methodology builds upon the existing large volume, mock Ly$α$ forest survey simulations presented by Greig et al. by including a prescription for a spatially inhomogeneous ionizing background, temperature fluctuations induced by patchy He${\rm\,{\scriptstyle II}}$ photoheating and the clustering of quasars. This approach enables us to achieve a dynamic range within our semi-analytic model substantially larger than currently feasible with computationally expensive, fully numerical simulations. The results agree well with existing numerical simulations, with large-scale temperature fluctuations introducing a scale-dependent increase in the spherically averaged 3D Ly$α$ forest power spectrum of up to 20-30 per cent at wavenumbers $k\sim0.02$ Mpc$^{-1}$. Although these large-scale thermal fluctuations will not substantially impact upon the recovery of the baryon acoustic oscillation scale from existing and forthcoming dark energy spectroscopic surveys, any complete forward modelling of the broad-band term in the Ly$α$ correlation function will none the less require their inclusion.