Lyα emission in high-redshift galaxies

Context. A significant fraction of the high-redshift galaxies show strong Ly alpha emission lines. For redshifts z > 5, most known galaxies belong to this class. However, so far not much is known about the physical structure and nature of these objects. Aims. Our aim is to analyse the Ly alpha emission in a sample of high-redshift UV-continuum selected galaxies and to derive the physical conditions that determine the Ly alpha profile and the line strength. Methods. VLT/FORS spectra with a resolution of R approximate to 2000 of 16 galaxies in the redshift range of z = 2.7 to 5 are presented. The observed Lya profiles are compared with theoretical models. Results. The Ly alpha lines range from pure absorption (EW = - 17 angstrom) to strong emission ( EW = 153 angstrom). Most Ly alpha emission lines show an asymmetric profile, and three galaxies have a double-peaked profile. Both types of profiles can be explained by a uniform model consisting of an expanding shell of neutral and ionised hydrogen around a compact starburst region. The broad, blueshifted, low-ionisation interstellar absorption lines indicate a galaxy-scale outflow of the ISM. The strengths of these lines are found to be determined in part by the velocity dispersion of the outflowing medium. We find star-formation rates of these galaxies ranging from SFRUV = 1.2 to 63.2 M-. yr(-1) uncorrected for dust absorption. Conclusions. The Ly alpha emission strength of our target galaxies is found to be determined by the amount of dust and the kinematics of the outflowing material.