Galaxies

Galaxies like our own Milky Way constantly evolve due to the interplay of the stars, the gas between the stars (the “interstellar medium”) and the galactic environment. Massive stars at the end of their lifetime explode violently and eject their synthesized elements like oxygen and iron into space, and newborn stars are formed out of this enriched gas. This stellar life cycle leads to younger stars being enriched in metals (note that astrophysicists have the weird convention of classifying all elements except hydrogen and helium as metals). Put another way, the galactic metal abundance generally increases over time. However, this doesn’t happen exactly the same way for all metals. Oxygen, for instance, is mostly produced by very massive stars which have a short lifetime (of the order of ten million years, which is short compared to the lifetime of the sun of ten billion years), while iron is mostly produced in a more complex process which requires a white dwarf – a stellar remnant which emerges not before hundred million years passed.

An interesting aspect of this is that one can use the oxygen-to-iron ratio in a galaxy as an indicator for the formation history. This chemical record is generally interpreted as follows: A galaxy with a high oxygen-to-iron ratio formed its stars in a short but vigorous burst of star formation, leading to enrichment in oxygen but there was no time to enrich the interstellar medium in iron. On the other hand, a galaxy with a low oxygen-to-iron ratio has a gentle and continuous star formation history, leading to a steady buildup of iron which decreases the galactic oxygen-to-iron ratio. We found that there are actually two ways to calculate the oxygen-to-iron ratios in galaxies, interestingly, they don’t coincide for the major fraction of galaxies which are still forming stars today. These two definitions provide complementary information about the galactic star-formation history, thereby extending the potential of the chemical composition as a fossil record.

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