The physics of the fundamental metallicity relation

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Abstract
We present a simple, redshift-independent analytic model that explains the local fundamental metallicity relation (FMR), taking into account the physical processes of star formation, inflow of metal-poor intergalactic medium (IGM) gas, and the outflow of metal-rich interstellar medium (ISM) gas. We show that the physics of the FMR can be summarized as follows: for massive galaxies with stellar mass M* ≥ 1011 M⊙, ISM metal enrichment due to star formation is compensated by inflow of metal-poor IGM gas, leading to a constant value of the gas metallicity with star formation rate (SFR); outflows are rendered negligible as a result of the large potential wells of these galaxies. On the other hand, as a result of their smaller SFR, less massive galaxies produce less heavy elements that are also more efficiently ejected due to their shallow potential wells; as a result, for a given M*, the gas metallicity decreases with SFR. For such galaxies, the outflow efficiency determines both the slope and the knee of the metallicity–SFR relation. Without changing any parameters, this simple model is also successfully matched to the gas fraction–gas metallicity relation observed for a sample of about 260 nearby galaxies.

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