3 Overview of EUV-UV spectra of starbursts at various metallicities

To illustrate several points discussed in detail below, we plot the EUV to UV spectral energy distribution (SED) for selected models at different metallicities and computed with different IMFs (see Fig. 2). Only ZAMS models are shown here for simplicity.

The main points apparent from this figure are:

• The rapid decrease of the He⁺ ionising photon flux (i.e. flux above 228 Å) and the associated overall softening of the ionising spectrum from Pop III to higher metallicity (cf. Sects. 4 and 6).
• The increase of the Lyman-break amplitude with increasing Z (Sect. 5).
• The rapid decrease of the contribution of nebular continuous emission to the UV continuum flux (cf. S02).
• The strong dependence of the ionising flux and in turn also of the nebular continuous emission on the upper mass cut-off of the IMF, which is considered as a free model parameter for very low ( $Z \le 10^{-5}$, currently unobserved) metallicities (Sects. 4 and 6).

The metallicity dependence of the emission lines (strongest in the Pop III models) cannot clearly be discerned from this figure. It is discussed in Sect. 7.