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

Galaxies at high redshift with a strong Ly-α 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-α 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-α emission redshifted into these windows, F_lim ≳ 3 × 10^-20 W m^-2, corresponds to (unobscured) star formation rates of ≳10 M_⊙/yr at z = 6. Candidate Ly-α-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-α-emitting galaxy candidates at z ≃ 4.8, and 11 candidates at z ≃ 5.7. For two of four observed Ly-α candidates, one candidate at z ≃ 4.8, and the other at z ≃ 5.7, the emission line detected with the Fabry-Perot interferometer has been verified spectroscopically at the VLT. When compared to Ly-α surveys at z ≤ 3.5, even the upper limits set by our list of candidates show that bright Ly-α galaxies are significantly rarer at z ≳ 5 than the assumption of a non-evolving population would predict. Therefore we conclude that the Ly-α bright phase of primeval star formation episodes reached its peak at redshifts 3 < z < 6.