The transition from population III to normal galaxies: Ly alpha and He II emission and the ionising properties of high redshift starburst galaxies

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 Lyalpha and He II lambda1640 recombination lines in starbursts. The metallicity dependence of these properties, especially the transition from primordial galaxies (Population 111) 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-. the enhancement reaches typically a factor of similar to3 between solar metallicity and Pop III objects. - While for metallicities Z greater than or similar to 1/50 Z(.) the amplitude of the Lyman-break depends little on Z, a reduction by a factor similar to2 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 similar to10(-4) the hardness decreases from log(Q(He+)/Q(H)) similar to -1.4... -2.3 by similar to1.5-2 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)) similar to -3.2 to -2.6 for metal-poor starbursts (1/25 less than or similar to Z/Z(.) less than or similar to 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 less than or similar to10(-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 Lyalpha equivalent width is shown to increase strongly with decreasing metallicity from W(Lyalpha) 250-350 Angstrom at Z 1/50 Z(.) to 400-850 Angstrom or higher at Z between 10(-5) and 0 (Pop III) for the same Salpeter IMF. However, for well known reasons, the Lyalpha 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(.)) less than or similar to -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.