Ly α forest power spectrum as an emerging window into the epoch of reionization and cosmic dawn

Conventional wisdom was that thermal relics from the epoch of reionization (EOR) would vanish swiftly. Recently, however, it was shown that these relics can survive to lower redshifts (z similar to 2) than previously thought, due to gas at mean density being heated to T similar to 3 x 10(4) K by reionization, which is inhomogeneous, and shocks. Given the high sensitivities of upcoming Ly alpha forest surveys, this effect will be a novel broad-band systematic for cosmological application. From the astrophysical point of view, however, the imprint of inhomogeneous reionization can shed light on the EOR and cosmic dawn. We utilize a hybrid method - which includes two different simulation codes capable of handling the huge dynamical range - to show the impact of patchy reionization on the Ly alpha forest and its dependence on different astrophysical scenarios. We found statistically significant deviations in the 1D Ly alpha power spectrum at k = 0.14 cMpc(-1) that range from similar to 1 per cent at z = 2 up to almost similar to 20 per cent at z = 4. The deviations in the 3D Ly alpha power spectrum, at the same wavenumber, are large and range from a few per cent at z = 2 up to similar to 50 per cent at z = 4, although these deviations ignore the effect of He II reionization and active galactic nucleus feedback at z < 4. By exploiting different k-dependence of power spectrum among various astrophysical scenarios, the effect of patchy reionization on the Ly alpha forest power spectrum can open a new window into cosmic reionization and possibly cosmic dawn.