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

TL;DR

This study measures the Lyman-alpha luminosity function at redshifts 5.7 and 6.6, revealing a steep faint-end slope and evidence for patchy reionization affecting faint emitters during the epoch of reionization.

Contribution

It provides the largest narrow-band survey of Lyman-alpha emitters at these redshifts, with new measurements of the luminosity function and insights into reionization's patchy nature.

Findings

Faint-end slope of the luminosity function is very steep at z=5.7.

Significant decline in faint Lyman-alpha emitters from z=5.7 to 6.6.

Extended halos around faint emitters at z=6.6 suggest increased scattering by neutral Hydrogen.

Abstract

We present new results from the widest narrow band survey search for Lyman-alpha (Lya) 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 Lya candidates at z=5.7 within a volume of 6.3x10$^6$ Mpc$^3$. Our Lya emitters span a wide range in Lya luminosities, from faint to bright (L$_{\rm Ly\alpha}\sim10^{42.5-44}$ erg s$^{-1}$) and rest-frame equivalent widths (EW$_0$~25-1000 \AA) in a single, homogeneous data-set. By combining all our fields we find that the faint end slope of the z=5.7 Lya luminosity function is very steep, with $\alpha=-2.3^{+0.4}_{-0.3}$. We also present an updated z=6.6 Lya 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 Lya emitters from z=5.7 to z=6.6 (by $0.5\pm0.1$ dex), but no evolution at the bright end/no evolution in L*. Faint Lya 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 Lya emission within the epoch of reionization. All together, our results suggest that we are observing patchy reionization which happens first around the brightest Lya emitters, allowing the number densities of those sources to remain unaffected by the increase of neutral Hydrogen fraction from z~5 to z~7.