SPECTROSCOPIC OBSERVATIONS OF LYMAN BREAK GALAXIES AT REDSHIFTS ∼4, 5, AND 6 IN THE GOODS-SOUTH FIELD

We report on observations of Lyman break galaxies (LBGs) selected from the Great Observatories Origins Deep Survey at mean redshifts z similar to 4, 5, and 6 (B435-, V606-, and i(775)-band dropouts, respectively), obtained with the red-sensitive FORS2 spectrograph at the ESO VLT. This program has yielded spectroscopic identifications for 114 galaxies (similar to 60% of the targeted sample), of which 51 are at z similar to 4, 31 at z similar to 5, and 32 at z similar to 6. We demonstrate that the adopted selection criteria are effective, identifying galaxies at the expected redshift with minimal foreground contamination. Of the 10% interlopers, 83% turn out to be Galactic stars. Once selection effects are properly accounted for, the rest-frame ultraviolet (UV) spectra of the higher redshift LBGs appear to be similar to their counterparts at z similar to 3. As at z similar to 3, LBGs at z similar to 4 and z similar to 5 are observed with Ly alpha both in emission and in absorption; when in absorption, strong interstellar lines are also observed in the spectra. The stacked spectra of Lya absorbers and emitters also show that the former have redder UV spectra and stronger but narrower interstellar lines, a fact also observed at z similar to 2 and 3. At z similar to 6, sensitivity issues bias our sample toward galaxies with Ly alpha in emission; nevertheless, these spectra appear to be similar to their lower redshift counterparts. As in other studies at similar redshifts, we find clear evidence that brighter LBGs tend to have weaker Ly alpha emission lines. At fixed rest-frame UV luminosity, the equivalent width of the Ly alpha emission line is larger at higher redshifts. At all redshifts where the measurements can be reliably made, the redshift of the Ly alpha emission line turns out to be larger than that of the interstellar absorption lines (ISLs), with a median velocity difference Delta V similar to 400 km s(-1) at z similar to 4 and 5, consistent with results at lower redshifts. This shows that powerful, large-scale winds are common at high redshift. In general, there is no strong correlation between the morphology of the UV light and the spectroscopic properties. However, galaxies with deep ISLs and strong Ly alpha absorption appear to be more diffuse than galaxies with Ly alpha in emission.