Since decades it's well known that a correlation between stellar mass and metallicity (Z; both stellar and gas-phase Z, but here I focus on the gas-phase Z) exists (e.g., Tremonti+04; analysis of >53000 galaxies from SDSS). More recent results have shown that the scatter of this "mass-Z" relation is mainly driven by the star formation rate (SFR; e.g., Mannucci+10), i.e., at a fixed stellar mass, galaxies with higher SFRs have lower Z. This happen systematically over a wide range of stellar masses.
What's the physical mechanism(s) responsible for that?

Also, even more recently (e.g., Bothwell+16) it has been shown that the "real" driver of the "mass-Z" relation's scatter is the molecular gas mass (the previously found dependence on SFR may be only a by-product of the Schmidt-Kennicutt relation between gas and SFR). How this would change the interpretation of the first problem?

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    $\begingroup$ This is an open research problem! There are still many papers being published on the topic. You could write a paper on the topic if you came up with a plausible explanation. $\endgroup$ – Doug Jun 14 '18 at 18:24
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    $\begingroup$ I would wonder here is if we could assume a correlation between Z and the "age" of a galaxy. I would think that larger galaxies would tend to be older (larger due to mergers which take time). Also, when comparing similar size galaxies I would think that those with higher SFR would be "younger" and thus have a lower Z. I could be way off track here and I am sure there are many anomalies that contradict this. $\endgroup$ – Jack R. Woods Jun 19 '18 at 16:52

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