The Velocity Map Asymmetry of Ionized Gas in MaNGA II. Correlation between Velocity Map Morphology, Star Formation, and Metallicity in Regular Disk Galaxies

TL;DR

This study uses MaNGA integral field spectroscopy to analyze ionized gas velocity map asymmetries in disk galaxies, revealing that external gas accretion influences kinematic asymmetry, metallicity, and star formation, with implications for galaxy evolution.

Contribution

It introduces a detailed analysis of velocity map asymmetries and their correlation with star formation and metallicity, highlighting external gas accretion as a key driver in galaxy kinematics.

Findings

Higher kinematic asymmetry correlates with deviations from scaling relations.

Galaxies with higher asymmetry tend to have lower metallicity and higher gas fractions.

External gas accretion likely causes increased asymmetry, metallicity dilution, and star formation enhancement.

Abstract

The morphology of ionized gas velocity maps provides a direct probe of the internal gas kinematics of galaxies. Using integral field spectroscopy from SDSS-IV MaNGA, we analyze a sample of 528 low-inclination, regular disk galaxies to investigate the correlations between velocity map morphology, star formation rate, and gas-phase metallicity. We quantify velocity map morphology using harmonic expansion and adopt two complementary diagnostics: the global kinematic asymmetry, which traces non-axisymmetric perturbations, and the first-order term ratio, which captures axisymmetric radial motions. We find that galaxies with higher kinematic asymmetry are more likely to deviate from the scaling relations, typically lying either above or below the star formation main sequence and systematically below the mass-metallicity relation. In contrast, the first-order term ratio shows only a correlation with gas-phase metallicity in the low-mass range and no significant dependence on star formation rate. Moreover, galaxies below the mass-metallicity relation generally exhibit higher HI gas fractions. These results suggest that external gas accretion is the primary driver of the observed phenomena: inflowing metal-poor gas increases velocity map asymmetry in disk galaxies, dilutes the metallicity, and triggers enhanced star formation. Feedback-driven outflows, bar- and spiral-driven inflows, and galaxy mergers may also contribute, but likely play a secondary role.