I will talk about the cycling of extragalactic gas into galaxies; its impact on star formation; the enrichment of the interstellar medium with oxygen produced in massive stars; and the explosion of these stars as Type II SNe that expel some of these oxygen-enriched gases back into extragalactic space. These different stages establish a chemical cycle whose components should quantitatively match.
Stars of different masses constituting the Initial Mass Function (IMF) of a star-forming episode eject oxygen and hydrogen in different ratios. To determine the expected amounts of oxygen generated in a typical star-forming episode we need to analyze the fractional oxygen abundance each stellar mass range of the IMF will yield, and how the net ratio of oxygen to hydrogen increases per star-forming episode. We also need to obtain estimates of the fraction of oxygen-rich Type II SNe ejecta escaping galaxies of different masses to enrich the oxygen abundance of extragalactic space, and thus also the metallicity of infalling gas ultimately contributing to the mix that initiates star formation in galaxies.
Much of the quantitative evidence supporting the cycling of gases into and out of galaxies can be obtained from four distinct sources: (i) the metallicities of nearly a million galaxies documented by the Sloan Digital Sky Survey, (ii) a quantitative analysis of metal-enriched ejecta returned to interstellar space by stars of distinct masses, (iii) the observed density of baryons in interstellar space and their fractional oxygen abundances, and (iv) the metallicities of massive galaxies apparently devoid of either infall or outflow.
An essential new concept I will introduce is that of a star-forming “duty cycle, Φ,” the fraction of its life that a galaxy is actively forming stars --- as contrasted to lying fallow, passively accumulating gas expelled in the winds of evolved low- and intermediate-mass stars.