Possible evidence for a large-scale enhancement in the Lyman-a forest power spectrum at redshift z=4

Inhomogeneous reionization enhances the 1D Ly alpha forest power spectrum on large scales at redshifts z >= 4. This is due to coherent fluctuations in the ionized hydrogen fraction that arise from large-scale variations in the post-reionization gas temperature, which fade as the gas cools. It is therefore possible to use these relic fluctuations to constrain inhomogeneous reionization with the power spectrum at wavenumbers log(10)( k/ km (-1) s ) ? -1 . 5. We use the Sherwood-Relics suite of hybrid radiation hydrodynamical simulations to perform a first analysis of new Ly alpha forest power spectrum measurements at 4.0 <= z <= 4.6. These data extend to wavenumbers log10( k/ km( -1) s )?-3, with a relative uncertainty of 10-20 per cent in each wavenumber bin. Our analysis returns a 2.7 a preference for an enhancement in the Ly alpha forest power spectrum at large scales, in excess of that expected for a spatially uniform ultraviolet background. This large-scale enhancement could be a signature of inhomogeneous reionization, although the statistical precision of these data is not yet sufficient for obtaining a robust detection of the relic post-reionization fluctuations. We show that future power spectrum measurements with relative uncertainties of ? 2.5 per cent should provide unambiguous evidence for an enhancement in the power spectrum on large scales.