  Integrated X-ray spectra of an evolving population of starburst galaxies
(SBGs) are determined based on the observed spectra of local SBGs. In addition
to emission from hot gas and binary systems, our model SBG spectrum includes a
nonthermal component from Compton scattering of relativistic electrons by the
intense ambient far-IR and the (steeply evolving) CMB radiation fields. We use
these integrated spectra to calculate the levels of contribution of SBGs to the
cosmic X-ray background assuming that their density evolves as (1+z)^q up to a
maximal redshift of 5. We find that at energies <10 keV this contribution is at
a level of few percent for q up to 3, and in the range of 5%-15% for q ~ 4.5.
The Compton component is predicted to be the main SBG emission at high
energies, and its relative contribution gets progressively higher for
increasing redshift.
