Spectroscopy of z ∼ 5 Lyman break galaxies in the ESO Remote Galaxy Survey

We present the global results of a large spectroscopic survey carried out in order to identify z similar to 5 Lyman break galaxies (LBGs) across 10 widely separated similar to 45 arcmin2 fields to a depth of I-AB = 26.3. The redshifts of seventy 4.6 < z < 5.6 LBGs were identified through their Ly alpha emission and/or a strong continuum break, with 38 sources showing detectable line emission of between 2.6 x 10-18 and 7 x 10-17 erg cm-2 s-1. Just over half of the spectroscopically confirmed z similar to 5 galaxies have rest-frame Ly alpha equivalent widths above 20 A, double the frequency of similarly strong line emitters in similar z similar to 3 LBG samples. However, when reasonable corrections are made for the spectroscopically unconfirmed sources that are nevertheless at these redshifts in both samples, we find no significant difference in the frequency of high equivalent width line emitters between the samples. The rest-frame UV continuum slope of a typical z similar to 5 line-emitting galaxy (as measured primarily from photometry, but also apparent in spectroscopy) is bluer than that of a typical break-only galaxy, a difference that is difficult to explain purely by differences in the ages of their stellar populations. Variation in metallicity and/or dust extinction can more straightforwardly account for this difference. If the correlation between metallicity and UV continuum slope identified at low redshift is applicable at z > 3, the typical z similar to 5 LBGs have metallicities a factor of 3 lower than those of LBGs at z similar to 3. Hubble Space Telescope imaging of a subset of the LBGs indicates that a large majority of the spectroscopically confirmed LBGs in our sample are members of multiple systems (on similar to arcsec scales) and/or show disturbed morphology. Using local LBG analogues as a model, this multiplicity could be explained either by super-starburst regions within a larger unseen structure or by a high incidence of merging events at this epoch. The current data cannot distinguish between these two possibilities. The surface density of z similar to 5 LBGs in two of the 10 fields is considerably higher than in the rest. Both show clear spikes in their redshift distributions indicating strong three-dimensional clustering in these fields. Against an expectation of about one source per 0.1 in redshift between 4.8 < z < 5.6, one field has seven identified objects between 5.11 < z < 5.21 and the other has 17 between 4.95 < z < 5.15. Neither structure can be bound given their depth in redshift and probably extend beyond the observed fields. The three-dimensional distances between LBGs in the structures are too large for them to have triggered their starbursts through mutual gravitational interaction, and so it is likely that the short-lived LBGs represent only a small fraction of the baryons in the structures.