  The new Pop III models by Schaerer (2002) have been used to derive the
observed properties of the first galaxies in terms of the expected magnitudes
and colors. The dependence of their properties on the IMF and upper mass limit
for star formation are studied. The emerging synthetic spectra are used to
discuss the implications on different observational features. Strong emission
lines, such as Lyman-alpha and HeII 1640, could easily be detected with a good
S/N with near-IR medium resolution spectrographs, such as EMIR at GTC. Our
simulations aim at exploring possible observational constraints on the
formation epoch of the first stars in galaxies.
