The clustering of Lyman alpha emitters at z=7: implications for reionization and host halo masses

Abstract: The Ly$\alpha$ line of high-redshift galaxies has emerged as a powerful probe of both early galaxy evolution and the epoch of reionization (EoR). Motivated by the upcoming wide-field survey with the Subaru Hyper Suprime-Cam (HSC), we study the angular correlation function (ACF) of narrow-band selected, $z\approx7$ Ly$\alpha$ emitting galaxies (LAEs). The clustering of LAEs is determined by both: (i) their typical host halo masses, $\bar{M}{\rm h}$; and (ii) the absorption due to a patchy EoR, characterized by an average neutral fraction of the intergalactic medium, $\bar{x}{\rm HI}$. We bracket the allowed LAE ACFs by exploring extreme scenarios for both the intrinsic Ly$\alpha$ emission and the large-scale pattern (i.e. morphology) of cosmic ionized patches in physical EoR models. Independent of the EoR morphology, current $z\approx 7$ ACF measurements constrain $\bar{x}{\rm HI}\lesssim 0.5$ (1-$\sigma$). We also find that the low values of the currently-observed ACF imply that LAEs are hosted by relatively small dark matter halos: $\bar{M}{\rm h} \lesssim 10^{10} M_{\odot}$, with corresponding duty cycles of $\lesssim$ few per cent. These values are over an order of magnitude lower than the analogous ones for color-selected, Lyman break galaxies, suggesting that $z\approx 7$ narrow-band LAEs searches are preferentially selecting young, star-burst galaxies, residing in less massive halos. The upcoming Ultra Deep campaign with the HSC will significantly improve constraints on both the EoR and LAE host halos.