Limits on the luminosity function of Ly alpha emitters at z=7.7

Aims. The Ly alpha luminosity function (LF) of high-redshift Ly alpha emitters (LAEs) is one of the few observables of the re-ionization epoch accessible with 8-10 m class telescopes. The evolution of the LAE LF with redshift is dependent upon the physical evolution of LAEs and the ionisation state of the Universe towards the end of the Dark Ages. Methods. We performed a narrow-band imaging program at 1.06 mu m using CFHT/WIRCam. The observations target Lya emitters at redshift z similar to 7.7 in the CFHT-LS D1 field. From these observations we derived a photometric sample of 7 LAE candidates at z similar to 7.7. Results. We derive luminosity functions for the full sample of seven objects and for subsamples of four objects. Assuming the brightest objects in our sample are real, we find that the resulting luminosity function is not consistent with previous work at lower redshifts. More definitive conclusions will require spectroscopic confirmation.