2003A&A...402...79M

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 700nm, 820nm, and 920nm. The typical flux detection limit for Ly-α emission redshifted into these windows, F_lim>3x10^–20W/m², corresponds to (unobscured) star formation rates of >10M_☉/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.