Context: Ly-alpha-emitters have proven to be excellent probes of faint, star-forming galaxies in the high redshift universe. However, although the sample of known emitters is increasingly growing, their nature (e.g. stellar masses, ages, metallicities, star-formation rates) is still poorly constrained. Aims: We aim to study the nature of Ly-alpha-emitters, to find the properties of a typical Ly-alpha-emitting galaxy and to compare these properties with the properties of other galaxies at similar redshift, in particular Lyman-break galaxies. Methods: We have performed narrow-band imaging at the VLT, focused on Ly-alpha at redshift z ~ 3.15, in the GOODS-S field. We have identified a sample of Ly-alpha-emitting candidates, and we have studied their Spectral Energy Distributions (SEDs). Results: We find that the emitters are best fit by an SED with low metallicity (Z/Z_sun = 0.005), low dust extinction (A_V ~ 0.32) and medium stellar masses of approximately 10^9 M_sun. The age is not very well constrained. One object out of 24 appears to be a high redshift Ly-alpha-emitting dusty starburst galaxy. We find filamentary structure as traced by the Ly-alpha-emitters at the 4 sigma level. The rest-frame UV SED of these galaxies is very similar to that of Lyman Break Galaxies (LBGs) and comply with the selection criteria for U-band drop-outs, except they are intrinsically fainter than the current limit for LBGs. Conclusion: Ly-alpha-emitters are excellent probes of galaxies in the distant universe, and represent a class of star-forming, dust and AGN free, medium mass objects.
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