Extracting the astrophysics of reionization from the Lyα forest power spectrum: a first forecast

The impact of cosmic reionization on the Ly alpha forest power spectrum has recently been shown to be significant even at low redshifts (z similar to 2). This memory of reionization survives cosmological time-scales because high-entropy mean-density gas is heated to similar to 3 x 10(4) K by reionization, which is inhomogeneous, and subsequent shocks from denser regions. In the near future, the first measurements of the Ly alpha forest 3D power spectrum will be very likely achieved by upcoming observational efforts such as the Dark Energy Spectroscopic Instrument (DESI). In addition to abundant cosmological information, these observations have the potential to extract the astrophysics of reionization from the Ly alpha forest. We forecast, for the first time, the accuracy with which the measurements of Ly alpha forest 3D power spectrum can place constraints on the reionization parameters with DESI. Specifically, we demonstrate that the constraints on the ionization efficiency, zeta, and the threshold mass for haloes that host ionizing sources, mturn, will have the la error at the level of zeta = 25.0 +/- 11.6 and log(10)(m(turn)/M-circle dot) = 8.7(-0.70)(+0.36), respectively. The Ly alpha forest 3D power spectrum will thus provide an independent probe of reionization, probably even earlier in detection with DESI, with a sensitivity only slightly worse than the upcoming 21-cm power spectrum measurement with the Hydrogen Epoch of Reionization Array (HERA), i.e. sigma(DESI)/sigma(HERA) approximate to 1.5 for zeta and sigma(DESI)/sigma(HERA) 2.0 for log(10)(m(turn)/M-circle dot). Nevertheless, the Ly alpha forest constraint will be improved about three times tighter than the current constraint from reionization observations with high-z galaxy priors, i.e. a sigma(DESI)/sigma(current) approximate to 1/3 for zeta.

Automated summary

The impact of cosmic reionization on the Ly alpha forest power spectrum is significant, and upcoming observational efforts like DESI will likely provide accurate measurements of reionization parameters through the Ly alpha forest 3D power spectrum. This measurement has the potential to be an independent probe of reionization, potentially even earlier than measurements with HERA.