The Lyα luminosity function at z=5.7-6.6 and the steep drop of the faint end: implications for reionization

We present new results from the widest narrow-band survey search for Ly alpha emitters at z = 5.7, just after reionization. We survey a total of 7 deg(2) spread over the COSMOS, UDS and SA22 fields. We find over 11 000 line emitters, out of which 514 are robust Ly alpha candidates at z = 5.7 within a volume of 6.3 x 10(6) Mpc(3). Our Ly alpha emitters span a wide range in Ly alpha luminosities, from faint to bright (L-Ly alpha similar to 10(42.5-44) erg s(-1)) and rest-frame equivalent widths (EW0 similar to 25-1000 angstrom) in a single, homogeneous data set. By combining all our fields, we find that the faint end slope of the z = 5.7 Ly alpha luminosity function is very steep, with alpha = -2.3(-0.3)(+0.4). We also present an updated z = 6.6 Ly alpha luminosity function, based on comparable volumes and obtained with the same methods, which we directly compare with that at z = 5.7. We find a significant decline of the number density of faint Ly alpha emitters from z = 5.7 to 6.6 (by 0.5 +/- 0.1 dex), but no evolution at the bright end/no evolution in L*. Faint Ly alpha emitters at z = 6.6 show much more extended haloes than those at z = 5.7, suggesting that neutral Hydrogen plays an important role, increasing the scattering and leading to observations missing faint Ly alpha emission within the epoch of reionization. Altogether, our results suggest that we are observing patchy reionization which happens first around the brightest Ly alpha emitters, allowing the number densities of those sources to remain unaffected by the increase of neutral Hydrogen fraction from z similar to 5 to 7.