The transition from Population III to normal galaxies: Lyα and He II emission and the ionising properties of high redshift starburst galaxies^*

Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14 Av. E. Belin, 31400 Toulouse, France

Corresponding author: schaerer@ast.obs-mip.fr

Received: 8 August 2002
Accepted: 11 October 2002

Abstract

Using new sets of stellar evolution models at very low metallicities (, 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: For young bursts the maximum Lyα equivalent width is shown to increase strongly with decreasing metallicity from Lyα –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 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 ( ) 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.