The extension of the Fundamental Metallicity Relation beyond the BPT star-forming sequence: evidence for both gas accretion and starvation

The fundamental metallicity relation (FMR) of galaxies is a 3D relation
between the gas-phase metallicity, stellar mass and star-formation rate (SFR).
It has been studied so far only for galaxies identified as star-forming (SF) on
the BPT diagrams (BPT-SF), but not for galaxies with LI(N)ER/AGN classification
(BPT-non-SF), mainly due to the lack of diagnostics for estimating their
gas-phase metallicities in the latter cases. We extend the FMR to BPT-non-SF
galaxies. To this end, we exploit the recent nebular line empirical
calibrations derived specifically for galaxies classified as non-SF in the BPT
diagrams. Moreover, we study an alternative representation of the FMR where we
consider the offsets in metallicity and SFR with respect to Main Sequence (MS)
galaxies. We find that galaxies with SFR higher than the MS are more metal-poor
than their counterparts on the MS, which is interpreted in terms of gas
accretion, boosting star formation and diluting the metallicity. Low-mass
galaxies below the MS (i.e. towards quiescence) have metallicities higher than
their MS counterparts, which is interpreted in terms of starvation, (i.e.
suppression of fresh gas supply) hampering star formation and reducing the
dilution effect, hence resulting in a higher level of internal chemical
enrichment. Massive galaxies below the MS have gas metallicity much closer to
their MS counterparts and much lower than expected from their stellar
metallicities; this result suggests a scenario where massive nearly-quiescent
galaxies with LI(N)ER-like nebular emission have recently accreted gas from the
circum/intergalactic medium.
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