How many Supernovae per galaxy per century are required to account for the current distribution of heavy elements found in stars?
First, it does not require a supernova to create elements heavier than iron. Heavy elements can also form in the cores of massive stars before they go supernova (s-process isotopes). Secondly, some elements beyond helium are formed in planetary nebulae. Some can also be formed through cosmic ray collisions. So the picture is a bit more complicated. Now for your questions. Galaxy metallicity (the fraction of heavy elements) can be derived from emission line spectroscopy of planetary nebulae and H-II regions in nearby spirals and irregulars, and from absorption line spectroscopy of large ensembles of stars in elliptical galaxies. The metallicities of galaxies depends on the star formation history (how many generations of supernova producing stars) and whether the newly synthesized metals can be retained. The latter depends mostly on mass (i.e., gravitational binding energy) — low mass galaxies lose a lot of metals in galactic winds. On getting metallicity of distant galaxies from optic
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