Predicted UV properties of very metal-poor starburst galaxies

Aims. We study the expected properties of starburst galaxies 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 He lambda 1640 line and two-photon continuum emission. Methods. 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. Results. The ionizing fluxes, UV continuum properties, and predicted Ly alpha and He lambda 1640 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 2 gamma 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 He lambda 1640 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 2 gamma continuum. Conclusions. 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.