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

TL;DR

This study finds that the inner HI gas mass in star-forming galaxies significantly influences the scatter in the mass-metallicity relation, highlighting the role of gas accretion over star formation rate in galaxy evolution.

Contribution

It introduces the inner HI mass as a key factor in reducing scatter in the mass-metallicity relation, emphasizing the importance of localized gas measurements in galaxy evolution models.

Findings

Inner HI mass correlates strongly with metallicity at fixed stellar mass.

Replacing total HI mass with inner HI mass reduces the scatter by 16%.

The correlation with inner HI mass weakens when metallicity is measured centrally.

Abstract

We use 789 disk-like, star-forming galaxies (with 596 HI detections) from HI follow-up observations for the SDSS-IV MaNGA survey to study the possible role of inner HI 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 HI mass witHIn the optical radius, but also use the total HI mass and star formation rate (SFR) for a comparison. We confirm the anticorrelation between the total HI mass and gas-phase metallicity at fixed stellar mass, but the anticorrelation is significantly strengthened when the total HI mass is replaced by the inner HI mass. Introducing a secondary relation with the inner HI 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 HI mass is robust when using different diagnostics of metallicity, but the correlation with SFR is not. The correlation with the inner HI 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 HI mass and gas-phase metallicity will provide new constraints for chemical and galaxy evolution models.