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 Lyα and He ii λ1640 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 Lyα and He ii λ1640 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 Lyα and 2γ 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 Lyα equivalent widths are therefore predicted at low metallicity. The He ii λ1640 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 2γ continuum. Conclusions. Our results have implications for the interpretation of star-forming metal-poor and/or high redshift galaxies, for galaxies among the Lyα emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of Population III stars in the distant Universe.

Context. The Great Observatories Origins Deep Survey (GOODS) has provided us with one of the deepest multi-wavelength views of the distant universe. The combination of multi-band photometry and optical spectroscopy has resulted in the identification of sources whose redshifts extend to values in excess of six. Amongst these distant sources are Lyα emitters whose nature must be deduced by clearly identifying the different components that contribute to the measured SED. Aims. From a sample of Lyα emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Lyα) that shows a second emission line, identified as N iv] 1486 Å. The SED is modelled in a way that accounts for both the N iv] line emission and the photometry in a self-consistent way.Methods. The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed N iv] luminosity and equivalent width.Results. A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of ∼5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong [O iii] 4959, 5007 Å emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 × 10 8 M . Conclusions. It is argued that objects with Lyα emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebulae with large telescopes will provide a unique insight into very early chemical evolution.