| Abstract
| - 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, $F_{\rm lim} \ga 3 \times 10^{-20} ~\rm{W}\,\rm{m}^{-2}$, corresponds to (unobscured) star formation rates of $\ga$$ 10\, {M}_{odot}$/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 $z \simeq 4.8$, and 11 candidates at $z \simeq 5.7$. For two of four observed Ly- α candidates, one candidate at $z \simeq 4.8$, and the other at $z \simeq 5.7$, the emission line detected with the Fabry-Perot interferometer has been verified spectroscopically at the VLT.
When compared to Ly- α surveys at $z\leq3.5$, even the upper limits set by our list of candidates show that bright Ly- α galaxies are significantly rarer at $z\ga 5$ 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 $3<z<6$.
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