Using new sets of stellar evolution models at very low metallicities ( Z = 10 ^ { -7 } , 10 ^ { -5 } ) and previously published grids we examine spectral properties of the ionising continua , the Lyman-break , and the Ly \alpha and He ii \lambda 1640 recombination lines in starbursts .
The metallicity dependence of these properties , especially the transition from primordial galaxies ( Population III ) to currently observed metallicities , is examined for various IMFs and star formation histories .

For the average properties of starbursts , approximated by a model with constant star formation , the main findings are :

• The Lyman continuum flux Q ( { H } ) increases with decreasing metallicity .
For a universal Salpeter IMF from 1–100 M _ { \odot } the enhancement reaches typically a factor of \sim 3 between solar metallicity and Pop III objects .

• While for metallicities Z \ga 1 / 50 Z _ { \odot } the amplitude of the Lyman-break depends little on Z , a reduction by a factor \sim 2 is found at lower metallicities , due to the strong increase of the average stellar temperature .

• Using theoretical models and empirical constraints we discuss the expected evolution of the hardness of He ^ { + } to H ionising photons , Q ( { He ^ { + } } ) / Q ( { H } ) , with metallicity and possible uncertainties .
Over the metallicity range Z = 0 to \sim 10 ^ { -4 } the hardness decreases from \log ( Q ( { He ^ { + } } ) / Q ( { H } ) ) \sim - 1.4 \ldots - 2.3 by \sim 1.5–2 or more orders of magnitude , depending strongly on the upper mass cut-off of the IMF .
From empirical constraints we derive a hardness \log ( Q ( { He ^ { + } } ) / Q ( { H } ) ) \sim –3.2 to –2.6 for metal-poor starbursts ( 1 / 25 \la Z / Z _ { \odot } \la 1 / 4 ) and softer spectra for higher metallicities .
We also provide a simple estimate of the possible impact of hot WR like stars on Q ( { He ^ { + } } ) / Q ( { H } ) at very low metallicities ( Z \la 10 ^ { -4 } ) .

• Calibrations for star formation rate determinations from various recombination lines at all metallicities and for various IMFs are derived .

For young bursts the maximum Ly \alpha equivalent width is shown to increase strongly with decreasing metallicity from W ( { Ly } \alpha ) \sim 250–350 Å at Z \ga 1/50 Z _ { \odot } to 400–850 Å or higher at Z between 10 ^ { -5 } and 0 ( Pop III ) for the same Salpeter IMF .
However , for well known reasons , the Ly \alpha emission predicted likely represents an upper limit .

Non-negligible He ii \lambda 1640 emission due to stellar photoionisation appears to be limited to very small metallicities ( \log ( Z / Z _ { \odot } ) \la - 5.3 ) and Population III objects .

The predictions , available on the Web through the CDS and at http : //webast.ast.obs-mip.fr/sfr/ , should be useful for a variety of studies regarding high redshift galaxies , cosmological reionisation , and others .