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 analyzes ionized-gas velocity-map morphology for 528 low-inclination MaNGA disk galaxies using harmonic-kinemetry to quantify non-circular motions via $v_{\text{asym}}$ and $v_{a_1/b_1}$. The results show that $v_{\text{asym}}$ correlates with both star formation rate and gas-phase metallicity across stellar masses, with more asymmetric maps associated with galaxies above or below the star formation main sequence and below the mass-metallicity relation. In contrast, $v_{a_1/b_1}$ mainly correlates with metallicity at $\log M_\ast<10.5$ and shows little dependence on SFR, suggesting different driving mechanisms for the two modes of non-circular motion. The authors argue that external gas accretion best explains the joint trends—inflows dilute metallicity, perturb velocity maps, and fuel star formation—though outflows, bar/spiral streaming, interactions, and warps may contribute but are secondary. Overall, the work provides a cohesive picture in which baryon cycling through accretion imprints measurable kinematic and chemical signatures on regular disk galaxies, with implications for understanding disk growth and the regulation of star formation.

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.

Figures (6)

• Figure 1: The relationship between stellar mass and velocity map morphology (Left: kinematic asymmetry $\log \overline{v}_{\text{asym}}$. Right: first-order term ratio $\log \overline{v}_{a_1/b_1}$). The red triangles represent the median value of velocity map morphology parameters within the given stellar mass bin. The dashed line is a linear fit to these median values as a function of stellar mass.
• Figure 2: Dependence of the star formation main sequence (SFMS, top) and the mass-metallicity relation (MZR, bottom) on velocity map morphology. In the left panels, colors represent $\Delta \log \overline{v}_{\text{asym}}$, while in the right panels, colors represent $\Delta \log \overline{v}_{a_1/b_1}$. The dashed lines mark the best-fitting trend of SFMS or MZR, and the dotted lines indicate the 25th and 75th percentiles.
• Figure 3: Cumulative distributions of $\Delta \log \overline{v}_{\text{asym}}$ (left) and $\Delta \log \overline{v}_{a_1/b_1}$ (right) for galaxies located at different positions relative to the SFMS (top panels) and the MZR (bottom panels). The cumulative distributions are constructed from the individual distribution functions of $\Delta \log \overline{v}_{\text{asym}}$ and $\Delta \log \overline{v}_{a_1/b_1}$ derived for each galaxy.
• Figure 4: Left: galaxies in the stellar mass -- sSFR plane, color-coded by their metallicity offset from the best-fit MZR. Right: galaxies in the stellar mass -- metallicity plane, color-coded by their SFR offset from the SFMS.
• Figure 5: The distribution of the offsets from the SFMS and MZR, where the colors represent $\Delta \log \overline{v}_{\text{asym}}$. The left and right panels display the results of low-mass galaxies ($\log M_\star<10.2$) and high-mass galaxies ($\log M_\star>10.2$), respectively.