2003A&A...397..527S

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 Lyα and He II λ1640 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: (o) [-] The Lyman continuum flux Q(H) increases with decreasing metallicity. For a universal Salpeter IMF from 1-100M_☉ the enhancement reaches typically a factor of ∼3 between solar metallicity and Pop III objects. (o) [-] While for metallicities Z>1/50Z_☉ the amplitude of the Lyman-break depends little on Z, a reduction by a factor ∼2 is found at lower metallicities, due to the strong increase of the average stellar temperature. (o) [-] 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 ∼10^–4 the hardness decreases from log(Q(He⁺)/Q(H)) ~-1.4 ... -2.3 by ∼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)) ∼ -3.2 to -2.6 for metal-poor starbursts (1/25 ≲Z/Z_☉ ≲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≲10^–4). (o) [-] Calibrations for star formation rate determinations from various recombination lines at all metallicities and for various IMFs are derived. For young bursts the maximum Lyα equivalent width is shown to increase strongly with decreasing metallicity from W(Lyα)∼250-350Å at Z>1/50Z_☉ to 400-850Å or higher at Z between 10^–5 and 0 (Pop III) for the same Salpeter IMF. However, for well known reasons, the Lyα emission predicted likely represents an upper limit. Non-negligible He II λ1640 emission due to stellar photoionisation appears to be limited to very small metallicities (log(Z/Z_☉)≲-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.