  We study the expected properties of starbursts in order to provide the point
of reference for interpretation of high-z galaxy surveys and of very metal-poor
galaxies. We concentrate mainly on the UV characteristics such as the ionizing
spectra, the UV continuum, the Ly alpha and HeII 1640 A line and two-photon
continuum emission. We use evolutionary synthesis models covering metallicities
from Pop III to solar and a wide range of IMFs. We also combine the synthetic
SEDs with the CLOUDY photoionization code for more accurate predictions of
nebular emission, and to study possible departures from case B assumed in the
synthesis models. The ionizing fluxes, UV continuum properties, and predicted
Ly alpha and HeII 1640 A line strengths are presented for synthesis models
covering a wider range of parameter space than our earlier calculations. Strong
departures from case B predictions are obtained for Ly alpha and two-photon
continuum at low metallicities. At low nebular densities both are shown to be
enhanced proportionally to the mean energy carried by the Lyman continuum
photons emitted by the ionizing source. Larger Ly alpha equivalent widths are
therefore predicted at low metallicity. The HeII 1640 A line can be weaker than
case B predicts (in terms of flux as well as the equivalent width) due to its
ionization parameter dependence and to the enhanced underlying two-photon
continuum. Our results have implications for the interpretation of star-forming
metal-poor and/or high redshift galaxies, for galaxies among the Ly alpha
emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of
Population III stars in the distant Universe.
