A&A 397, 527-538 (2003)
The transition from Population III to normal galaxies: Ly and He II emission and the ionising properties of high redshift starburst galaxiesD. Schaerer
Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14 Av. E. Belin, 31400 Toulouse, France
(Received 8 August 2002 / Accepted 11 October 2002 )
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 and He II 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: For young bursts the maximum Ly equivalent width is shown to increase strongly with decreasing metallicity from W(Ly -350 Å at 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 emission predicted likely represents an upper limit. Non-negligible He II 1640 emission due to stellar photoionisation appears to be limited to very small metallicities ( ) 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.
Key words: cosmology: early Universe -- galaxies: stellar content -- stars: general -- stars: fundamental parameters -- stars: atmospheres
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