Exploring 21 cm-Lyα Emitter Synergies for SKA

We study the signatures of reionization and ionizing properties of early galaxies in the cross-correlations between the 21 cm emission from the spin-flip transition of neutral hydrogen (H. I) and the underlying galaxy population. In particular, we focus on a sub-population of galaxies visible as Ly alpha Emitters (LAEs). With both observables simultaneously derived from a z similar or equal to 6.6 hydrodynamical simulation (GADGET-2) snapshot post-processed with a radiative transfer code (pCRASH) and a dust model, we perform a parameter study and aim to constrain both the average intergalactic medium (IGM) ionization state (1 -<chi(HI)>) and the reionization topology (outside-in versus inside-out). We find that, in our model, LAEs occupy the densest and most-ionized regions resulting in a very strong anti-correlation between the LAEs and the 21 cm emission. A 1000 hr Square Kilometer Array (SKA)LOW1-Subaru Hyper Suprime-Cam experiment can provide constraints on <chi(HI)>, allowing us to distinguish between IGM ionization levels of 50%, 25%, 10%, and fully ionized at scales r less than or similar to 10 comoving Mpc (assuming foreground avoidance for SKA). Our results support the inside-out reionization scenario where the densest knots (under-dense voids) are ionized first (last) for <chi(HI)>. 0.1. Further, 1000 hr SKA-LOW1 observations should be able to confirm the inside-out scenario by detecting a lower 21 cm brightness temperature (by about 2-10 mK) in the densest regions (greater than or similar to 2 arcmin scales) hosting LAEs, compared to lower-density regions devoid of them.