Dynamical Origins of Azimuthal Metallicity Variations in the Galactic Disk: Insights from Kinematic Ridges with Gaia

TL;DR

This study tests whether azimuthal metallicity variations in the Milky Way’s disk share a dynamical origin with Gaia-observed kinematic ridges. Using Gaia DR3 thin-disk data and Milky Way–like test-particle simulations that include a bar, transient spiral arms, and a Sagittarius-like satellite, the authors assess which perturbations reproduce the observed chemo-kinematic patterns. They find that models with bar + transient spiral arms best match the alignment between metallicity substructures and velocity-space ridges, while Sagittarius-like perturbations produce weaker azimuthal variations. The results support a dynamical origin for azimuthal metallicity variations and link them to the same processes that generate kinematic ridges and co-moving groups in the Galactic disk.

Abstract

Kinematic and spectroscopic studies in the past few years have revealed coherent azimuthal metallicity variations across the Milky Way's disk that may be the result of dynamical processes associated with non-axisymmetric features of the Galaxy. At the same time, stellar kinematics from Gaia have uncovered ridge-like features in the velocity space, raising the question of whether these chemical and dynamical substructures share a common origin. Using a sample of disk stars from Gaia DR3, we find that azimuthal metallicity variations are correlated with kinematic ridges in the V_phi-R plane, suggesting a shared origin. We utilize a suite of Milky Way test-particle simulations to assess the role of transient spiral arms, the bar, and interactions with a Sagittarius-like dwarf galaxy in simultaneously shaping both chemical and kinematic substructures. Among the physical mechanisms explored, bar and spiral arm interactions are the ones that consistently reproduce both the chemo-kinematic features and alignment observed in the Gaia data. While our model of an interaction with a Sagittarius-like dwarf galaxy can also induce kinematic and metallicity substructure, the amplitude of the azimuthal metallicity variations are too weak, suggesting this is likely not the dominant influence. Although additional contributing processes cannot be ruled out, the azimuthal metallicity variations observed in Gaia are best explained by a dynamical origin. Our results support the view that that azimuthal metallicity variations in the Galaxy are driven by similar dynamical mechanisms responsible for generating the kinematic ridges and co-moving groups.

Figures (9)

• Figure 1: Kiel Diagram of the full Gradient Analysis Sample stars. The grey box represents the subsample of stars that fulfill the $\log g$ and $T_\mathrm{eff}$ constraints in the text of Section \ref{['sec:data']}.
• Figure 2: [M/H] distribution of our stellar thin disk sample as a function of the star's guiding center radius. The white circles represent the median [M/H] of all stars in $0.2$ kpc bins with the black vertical bars spanning the 16th-84th percentile range. The best-fitting linear function is overlaid as a solid black line from 5kpc to 11kpc.
• Figure 3: Left Panel: [M/H] distribution of stars in the thin disk sample, plotted in the X-Y galactocentric coordinates. Middle Panel: Best-fitting radial metallicity profile in the X-Y plane. Right Panel: Metallicity Excess ($\delta [M/H]_{R_G}$; Data-Model) in the X-Y plane. The black dashed curves represent circles of radius 6kpc, 8kpc, 10kpc, and 12kpc. The yellow star denotes the sun's location in the X-Y plane.
• Figure 4: Top Panel: Distribution of azimuthal velocity ($V_{\phi}$) as a function of galactocentric radius (R), colored by number density. Upper Middle Panel: Same as top panel but colored by radial velocity. Overlaid in this panel are the approximate locations of the local co-moving groups. The two black dot-dash lines indicate the slopes of the kinematic ridges associated with the Hercules and Sirius co-moving groups. Lower Middle Panel: Same as top panel but colored by $\delta [M/H]_{R_G}$. Bottom Panel:$\delta [M/H]_{R_G}$ in the X-Y plane. The black dashed curves represent circles of radius 6kpc, 8kpc, and 10kpc. The yellow star denotes the sun's location in the XY plane.
• Figure 5: First Column: Distribution of azimuthal velocity ($V_{\phi}$) as a function of galactocentric radius (R), colored by number density. Upper Middle Panel: Same as top panel but colored by galactocentric radial velocity. The two black dot-dashed lines indicate the slopes of the kinematic ridges associated with the Hercules and Sirius co-moving groups. Lower Middle Panel: Same as top panel but colored by [M/H] excess and overlaid with the dot-dashed lines from the upper middle panel. Bottom Panel: Metallicity Excess in the X-Y plane. The black dashed curves represent circles of radius 6kpc, 8kpc, 10kpc, and 12kpc. The yellow star denotes the sun's location in the XY plane. Subsequent Columns: The same set of plots for the first column but for each set of MW simulations at the present-day snapshot (See Section \ref{['sec:present-day']} for details on selecting the present-day snapshot). Subplots are labeled A-T solely for convenience when referring to them in the text.