1 Introduction

The discovery of numerous high redshift galaxies provides a unique opportunity to study galaxies in formation in the early Universe. Most of these galaxies show signs of actively ongoing massive star formation, as revealed by their overall spectral appearance, by detailed spectral features, and in many cases by the presence of strong emission lines.

In fact, since the detection of Lyman-break galaxies at $z \sim$ 2-4 by colour selection techniques (Steidel et al. 1996; review by Stern & Spinrad 1999), Ly$\alpha$ surveys or other search techniques have found a large number of objects at higher redshift showing in most cases intense line emission (e.g. Hu et al. 1998, 1999; Kudritzki et al. 2000; Rhoads & Malhotra 2001; Malhotra & Rhoads 2002; Ellis et al. 2002; Frye et al. 2002; Ajiki et al. 2002). This also includes the most distant galaxy known to date, a lensed galaxy at z=6.56 found through its Ly$\alpha$ emission (Hu et al. 2002).

It is possible that such strong line emitters showing also relatively little continuum light may represent the earliest stages of galaxy formation, where small amounts of metals have so far been formed. Strong ongoing star formation and a small dust content, which can suppress Ly$\alpha$ emission, would then explain the high observed Ly$\alpha$ equivalent widths (cf. Hu et al. 1998). More striking is the suggestion of Malhotra & Rhoads (2002) that the high Ly$\alpha$ equivalent widths observed in the LALA survey at z=4.5 could, among other explanations, be due to metal-free (so-called Population III, hereafter Pop III) objects. Possibly we are beginning to probe distant chemically little evolved galaxies, closing the gap between the first (primordial) galaxies and the high (close to solar) metallicities of massive galaxies in the local Universe.

To properly study these objects appropriate spectral models are necessary. They should take into account all possible metallicities, and also probable systematic changes of the stellar initial mass function (IMF) at very low metallicity (Abel et al. 1998; Bromm et al. 1999; Nakamura & Umemura 2001; Hernandez & Ferrara 2001). Providing such model calculations is the main aim of the present work.

Other applications also require an understanding of how properties like line emission and the ionising fluxes of starburst behave in the transition between metal-free (Pop III) objects and metal-poor galaxies with observable counterparts in the local Universe. This is the case in studies addressing the re-ionisation history of the Universe (e.g. Gnedin 1998; Ciardi et al. 2000; review by Loeb & Barkana 2000), especially if account is taken for the simultaneous metal-enrichment (cf. Gnedin & Ostriker 1997; Ferrara & Schaerer 2002). Also, for searches of primordial galaxies, it is of interest to explore how far extreme properties such as strong He  II emission predicted for Pop III starbursts (Tumlinson & Shull 2000; Tumlinson et al. 2001, 2002; Oh et al. 2001; Bromm et al. 2001b; Schaerer 2002) are truly limited to zero metallicity. Our model calculations, including in particular metallicities Z=0, 10^-7, 10^-5, $4 \times 10^{-4}$ and higher, allow, for the first time, such investigations.

The present paper is structured as follows. Our models ingredients, including two new sets of stellar evolution tracks at very low metallicity, are described in Sect. 2. The predicted Lyman continuum fluxes and the properties of the Lyman-break at all metallicities are presented in Sects. 4 and 5 respectively. In Sect. 6 we discuss theoretical predictions and empirical constraints on the He⁺ ionising flux and the hardness of the ionising spectra of starbursts at various metallicities. Finally, quantitative predictions for the Ly$\alpha$ and He  II $\lambda$1640 emission are given in Sect. 7. Section 8 summarises our main conclusions.