Global chemical evolution - II. The mean metal abundance of the Universe

We examine the metallicity distribution within and between galaxies and hence determine their present-day mean abundance. Adding in components for intergalactic gas, we arrive at an estimate of the mean metal abundance of the Universe. We find the pleasantly Copernican result that the overall mean abundance is dose to solar. We then discuss the evolution with time of this quantity, using simplified, but general, models of galactic chemical evolution. The variation of the total metal content of the Universe with epoch turns out to be constrained within fairly well-defined limits for plausible variations in the global mean star formation rate. However, what would be observed at any given redshift depends critically on which regions of galaxies are being sampled and on the formation histories of these particular regions. Finally, we investigate the joint constraints provided by current observations of the evolution of the metallicity and gas content of the Universe, as measured by QSO damped Lyman a absorbers. We note a generic inconsistency in global models and introduce a more realistic model, with different evolution for different galaxy types, which can overcome this difficulty. Current chemical evidence does not require that the global average star formation rate in the Universe should have decreased by a large factor since galaxy formation began.