The Role of Inner H I Mass in Regulating the Scatter of the Mass-Metallicity Relation

We use 789 disk-like, star-forming galaxies (with 596 H I detections) from H I follow-up observations for the SDSS-IV MaNGA survey to study the possible role of inner H I gas in causing secondary dependences in the mass-gas-phase metallicity relation. We use the gas-phase metallicity derived at the effective radii of the galaxies. We derive the inner H I mass within the optical radius, but also use the total H I mass and star formation rate (SFR) for a comparison. We confirm the anticorrelation between the total H I mass and gas-phase metallicity at fixed stellar mass, but the anticorrelation is significantly strengthened when the total H I mass is replaced by the inner H I mass. Introducing a secondary relation with the inner H I mass can produce a small but noticeable decrease (16%) in the scatter of the mass-gas-phase metallicity relation, in contrast to the negligible effect with the SFR. The correlation with the inner H I mass is robust when using different diagnostics of metallicity, but the correlation with SFR is not. The correlation with the inner H I mass becomes much weaker when the gas-phase metallicity is derived in the central region instead of at the effective radius. These results support the idea that the scatter in the mass-metallicity relation is regulated by gas accretion, and not directly by the SFR, and stress the importance of deriving the gas mass and the metallicity from roughly the same region. The new relation between inner H I mass and gas-phase metallicity will provide new constraints for chemical and galaxy evolution models.