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We present a unified model for the detailed chemical evolution of individual elements from 12C to 56Fe and the photometric properties of galaxies via spectral evolutionary synthesis. Observations of narrow, heavy - element QSO absorption lines show an increase in the number of MgII systems per redshift interval for redshifts 0 <Zr ≤ 1.5 and a decrease in the number of CIV systems for 1.3 ≤ Zr ≤4.1. Both can be understood in terms of our galaxy evolution model accounting for SNI contributions which at the same time gives information about the structure of the Universe and about the IMF and star formation history in the intervening absorber galaxies. The spectrophotometric aspect of our unified model predicts spectral and photometric properties of these galaxies testable by optical identifications.
Evolutionary synthesis models have been computed to construct the spectral energy distributions of BCD galaxies in the optical and NIR ranges (0.3 to 3.5μm). Evolutionary tracks for stars having Z = 1/10 Z⊙ have been employed in order to match the observed low metal abundances of BCDs. Gaseous emission from H II regions has been included in the model. A starburst (of duration 5 · 106yr) is superimposed on an underlying component of red stars characterised by continuous star formation. Burst parameters, star formation rates and ionised hydrogen gas masses have been deduced by fitting the models to observed spectral energy distributions (SEDs) of BCDs.
A survey of soft X-ray background observations in the 0.1–10 keV range is presented. In the region above 1 keV, recent results on point X-ray sources are discussed and their integrated contribution to the diffuse background is estimated. However, the average luminosity of various classes of extragalactic X-ray sources is still not sufficiently well known to permit this estimate to be made with any certainty. A discussion is given of recent observations at energies below 1 keV where the effects of interstellar absorption are important. It is argued that although some fraction of the background radiation in the 0.1–1 keV range must be galactic in origin, there is still substantial evidence for an extragalactic component. Proposed theories for generating both the galactic and extragalactic X-ray background are briefly reviewed.