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Gas fraction (or gas mass fraction or baryonic gas fraction) is the ratio (by mass) of gas within a galaxy or galaxy cluster to all the baryonic mass, i.e., also including the galaxy or cluster's stars.
Mgas
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Mgas + Mstars
The term molecular gas fraction is similar, but indicates the gas mass in the numerator is specifically gas in molecular form, i.e., ready for star formation.