has all these properties: in particular we took advantage of the excellent observations in the near-IR, which cover the rest-frame region around the 4000 Å Balmer break, reducing some of the model degeneracies. Furthermore, the deep IRAC data allow the determination of stellar masses with great accuracy (e.g. Fontana et al. 2006).\
In previous works, we have compared the properties of LBGs with and without line emission for a relatively small sample of z$\sim$ 4 galaxies (Pentericci et al. 2007, hereafter P07), and then studied the properties of a more numerous sample of Ly$\alpha$ emitting LBGs in the redshift range 3 to 6 (Pentericci et al. 2009, hereafter P09). In this paper we study the properties of LBGs at redshift $\sim$2.5-3.5, the so-called U–dropouts, initially selected from their continuum and color properties and with follow-up spectroscopic confirmation: the lower redshift allows us to assemble a sample much larger than in our previous work and study the trends in a statistically more significant way. At the same time thanks to the brighter average magnitude of the galaxies we can also attempt a morphological analysis, which was not possible at higher z.\
As in previous work, we use the GOODS–MUSIC catalog (Grazian 