The physical properties of Lyα emitting galaxies: not just primeval galaxies?

Aims. We have analyzed a sample of Lyman break galaxies from z similar to 3.5 to z similar to 6 selected from the GOODS-S field as B, V, and i-dropouts, and with spectroscopic observations showing that they have the Ly alpha line in emission. Our main aim is to investigate their physical properties and their dependence on the emission line characteristic and to shed light on the relation between galaxies with Lya emission and the general LBG population. Methods. The objects were selected from their optical continuum colors and then spectroscopically confirmed by the GOODS collaboration and other campaigns. From the public spectra we derived the main properties of the Ly alpha emission such as total flux and rest frame EW. We then used complete photometry, from U band to mid-infrared from the GOODS-MUSIC database, and through standard spectro-photometric techniques we derived the physical properties of the galaxies, such as total stellar mass, stellar ages, star formation rates, and dust content. Finally we investigated the relation between emission line and physical properties. Results. Although most galaxies are fit by young stellar populations, a small but non negligible fraction has SEDs that cannot be represented well by young models and require considerably older stellar component, up to similar to 1 Gyr. There is no apparent relation between age and EW: some of the oldest galaxies have high line EW, and should be also selected in narrow-band surveys. Therefore not all Ly alpha emitting galaxies are primeval galaxies in the very early stages of formation, as is commonly assumed. We also find a range of stellar populations, with masses from 5 x 10(8) M(circle dot) to 5 x 10(10) M(circle dot) and SFR from few to 60 M(circle dot) yr(-1). Although there is no net correlation between mass and EW, we find a significant lack of massive galaxies with high EW, which could be explained if the most massive galaxies were either dustier and/or if they contained more neutral gas than less massive objects. Finally we find that more than half of the galaxies contain small but non negligible amounts of dust: the mean E( B-V) derived from the SED fit and the EW are well-correlated, although with a large scatter, as already found at lower redshift.