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Which physical quantity is the real / definitive tracer of galaxy evolution?

Let me explain better: if you aimed at understanding as much as possible about the evolution of galaxies and you had to choose only one quantity to perfectly follow/observe (directly/indirectly) across the cosmic time, which quantity you would chose? why? Is it possible? If not, why? What combination of quantities you would choose?

Stellar Mass, SFR, Metallicity, Dark Matter Mass, Angular momentum...

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  • $\begingroup$ Why only one? Why are you asking this question? BTW The question smells like homework and we're not doing that for you on SE sites. $\endgroup$ – user1569 Oct 25 '17 at 15:27
  • $\begingroup$ I asked about preferably one but, as specified, I am curious also about a combination of minimal quantities, along with explanations/interpretations. It is just curiosity... something to think/speculate about, no homework :) (I guess it would be a difficult questions to ask as homework!). Feel free to think about and give your opinion if you want... otherwise, no. $\endgroup$ – mic-jordan Oct 25 '17 at 16:21
  • $\begingroup$ Opinion-based questions is also not really desired in this site. It's a bit like asking "How would you characterize a person with one property? Height, weight, sense of humor, or favorite Alan Parker movie?" $\endgroup$ – pela Oct 25 '17 at 20:20
  • $\begingroup$ 1) It is not an opinion-based question, as I am looking for (and asking about) scientific argumentations of the answer. 2) The question "How would you characterise a person with one property?" would be a appropriate question for a sociology SE site, as well as "Which physical quantity is the real/definitive tracer of galaxy evolution?" is an appropriate question to share with an astronomers community. Anyway, still got no serious answers. It seems like it's easier complaining about the game's rule than playing the game. $\endgroup$ – mic-jordan Oct 25 '17 at 21:02
  • $\begingroup$ Your comment says "combination of minimal quantities" whereas your questions says "only one quantity". This is a contradiction and makes your question unclear. $\endgroup$ – StephenG Oct 26 '17 at 2:55