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 \approx 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 _ { \odot } = 0.005 ) , low dust extinction ( A _ { V } \approx 0.32 ) and medium stellar masses of approximately 10 ^ { 9 } M _ { \odot } .
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 .

Conclusions : 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 .