Constraints to the evolution of Ly-α bright galaxies between z=3 and z=6

Galaxies at high redshift with a strong Ly-alpha emission line trace massive star formation in the absence of dust, and can therefore be regarded as a prime signature of the first major starburst in galaxies. We report results of the Ly-alpha search within the Calar Alto Deep Imaging Survey (CADIS). With the imaging Fabry-Perot interferometer, CADIS can detect emission lines in three waveband windows free of night-sky emission lines at 700 nm, 820 nm, and 920 nm. The typical flux detection limit for Ly-alpha emission redshifted into these windows, F(lim)greater than or similar to3x10(-20) W m(-2), corresponds to (unobscured) star formation rates of greater than or similar to10 M-circle dot/yr at z=6. Candidate Ly-alpha-emitting galaxies are selected from the total emission line sample, which contains more than 97% of objects at z<1.2, by the absence of flux below the Lyman limit (B-band dropouts), and the non-detection of secondary emission lines in narrow band filters. We have detected 5 bright Ly-alpha-emitting galaxy candidates at z similar or equal to 4.8, and 11 candidates at z similar or equal to 5.7. For two of four observed Ly-alpha candidates, one candidate at z similar or equal to 4.8, and the other at z similar or equal to 5.7, the emission line detected with the Fabry-Perot interferometer has been verified spectroscopically at the VLT. When compared to Ly-alpha surveys at z <= 3.5, even the upper limits set by our list of candidates show that bright Ly-alpha galaxies are significantly rarer at z greater than or similar to 5 than the assumption of a non-evolving population would predict. Therefore we conclude that the Ly-alpha bright phase of primeval star formation episodes reached its peak at redshifts 3<6.