Lyα emission in high-redshift galaxies

Abstract

Context: A significant fraction of the high-redshift galaxies show strong Lyα 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 Lyα emission in a sample of high-redshift UV-continuum selected galaxies and to derive the physical conditions that determine the Lyα profile and the line strength.
Methods: VLT/FORS spectra with a resolution of R ≈ 2000 of 16 galaxies in the redshift range of z = 2.7 to 5 are presented. The observed Lyα profiles are compared with theoretical models.
Results: The Lyα lines range from pure absorption (EW = -17 Å) to strong emission (EW = 153 Å). Most Lyα 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 SFR_{UV} = 1.2 to 63.2 M_⊙yr^-1 uncorrected for dust absorption.
Conclusions: The Lyα emission strength of our target galaxies is found to be determined by the amount of dust and the kinematics of the outflowing material.

Based on observations

(proposals: 69.A-0105(A) and 71.A-0307(A)) obtained at the ESO VLT at

Cerro Paranal, Chile, and on observations made with HST ACS (GO proposal:

9502).