Lyα emission galaxies at a redshift of z ≈ 5.7 in the FORS deep field

Context.We present the results of a search for ${\rm Ly}{\alpha}$ emission galaxies at $z\approx5.7$ in the FORS Deep Field. Aims.The objective of this study is to improve the faint end of the luminosity function of high-redshift ${\rm Ly}{\alpha}$ emitting galaxies and to derive properties of intrinsically faint ${\rm Ly}{\alpha}$ emission galaxies in the young universe. Methods.Using FORS2 at the ESO VLT and a set of special interference filters, we identified candidates for high-redshift ${\rm Ly}{\alpha}$ galaxies. We then used FORS2 in spectroscopic mode to verify the identifications and to study their spectral properties. Results.The narrow-band photometry resulted in the detection of 15 likely ${\rm Ly}{\alpha}$ emission galaxies. Spectra with an adequate exposure time could be obtained for eight galaxies. In all these cases the presence of ${\rm Ly}{\alpha}$ emission at $z \approx 5.7$ was confirmed spectroscopically. The line fluxes of the 15 candidates range between 3 $\times$ 10-21 W m-2 and 16 $\times$ 10-21 W m-2, which corresponds to star-formation rates uncorrected for dust absorption between 1 and 5 ${M}_{\odot}$ yr-1. The luminosity function derived for our photometrically identified objects extends the published luminosity functions of intrinsically brighter ${\rm Ly}{\alpha}$ galaxies. Conclusions.With this technique the study of high-redshift ${\rm Ly}{\alpha}$ emission galaxies can be extended to low intrinsic luminosities.