Constraining the astrophysics of reionization with DESI


The impact of reionization on the Lyman-alpha forest power spectrum has recently been shown to be quite significant even at low redshifts (z~2). This memory of reionization survives cosmological time scales because of i) the patchy nature of reionization and ii) underdense regions (i.e. gas in minivoids) which reionize to high entropy and are later compressed (and heated to even larger temperatures) by shocks from denser regions. This effect opens a novel window into the astrophysics of reionization from the post-reionization Universe, which will become online as soon as DESI achieves the near milestone of the first measurement of the 3D Lyman-alpha power spectrum. Here, we forecast, for the first time, the ability of DESI to extract the astrophysics of reionization from the Lyman alpha forest power spectrum. In particular, we found that the 1-sigma errors are 12.2 and 6.91 for the ionization efficiency and threshold mass, respectively. These errors are competitive (but reasonably larger than) previously forecasted errors on the same parameters by using a mock 1000h observation with HERA and SKA. Furthermore, we demonstrate the previously unforeseen ability of DESI to infer the global reionization history, which is naturally worse compared to instruments designed to probe the Epoch of Reionization. Although the performance of DESI is inferior, the prospect for rich cross-correlations due to different systematics is exciting. Besides, DESI may measure the 3D Lyman-alpha power spectrum before the 21cm power spectrum is measured.