Constraints on Radial Gas Flows in the Milky Way Disk Revealed by Large Stellar Age Catalogs

TL;DR

This work tests whether radial gas flows in the Milky Way disk shape chemical evolution by examining five $v_{r,g}$ prescriptions within a multi-zone GCE framework, anchored by a large, precise stellar age-metallicity dataset. It demonstrates that inward flows dilute outer regions and steepen gradients, while potentially driving rapid equilibration in the outer disk, but cannot alone erase the age-metallicity trend across all radii. A constant inward velocity of about $-1$ km s$^{-1}$ in the outer disk provides the best alignment with outer-disk constraints, yet mass-conservation and inner-disk pile-up prevent flows from fully reconciling observations everywhere. The authors derive analytic gradient-evolution formulas and equilibration timescales, linking radial flow prescriptions to $\nabla[\mathrm{O/H}]_{eq}$ and $\tau_{eq}$, offering practical tools to diagnose accretion distributions and the role of angular momentum transport in MW-like galaxies.

Abstract

Disk galaxies like the Milky Way are expected to experience gas flows carrying matter toward their centers. This paper investigates the role of these radial gas flows in models of Galactic chemical evolution (GCE). We follow five different parameterizations of the Galactocentric radial velocity, $v_{r,g}$, of the interstellar medium (ISM). Relative to the $v_{r,g}=0$ limit, all models predict stellar metallicity to decline less significantly with age in the outer disk and more significantly in the inner disk. This outcome arises because radial flows cannot remove gas from one region of the Galaxy without placing it elsewhere, leading to opposing effects on enrichment timescales between the inner and outer Galaxy. This prediction is at odds with recent observational constraints, which indicate remarkably minimal decline in metallicity ($\lesssim$$0.1$ dex) between young ($\sim$$0-2$ Gyr) and old populations ($\sim$$8-10$ Gyr) across the \textit{entire} Galactic disk. Radial gas flows cannot be the sole explanation of this result at all Galactocentric radii. Our models reproduce this result at $R\gtrsim6$ kpc if the flow velocity is relatively constant in both radius and time near $v_{r,g}\approx-1$ km/s. In agreement with previous GCE models, all of our flow prescriptions lead to lower metallicities and steeper radial gradients relative to static models. Exploiting this universal outcome, we identify mixing effects and the relative rates of star formation and metal-poor accretion as the processes that establish the ISM metallicity at low redshift. We provide a suite of analytic formulae describing radial metallicity gradient evolution based on this connection.

Figures (17)

• Figure 1: Distribution of observed disk radial velocities, averaged across radius in individual external galaxies, measured using doppler shifts in the H i 21 cm line by DiTeodoro2021. They define $R_{25}$ as the radius corresponding to the 25 mag/arcsec$^2$ isophote in the $z$ band and $R_\text{H i}$ as the radius at which the projected H i surface density drops below 1 $\text{M}_\odot$/pc$^2$. Summary: Observed Galactocentric radial velocities are highly variable both between and within galaxies, which simulations suggest is a consequence of variations on short timescales ($\ll 1$ Gyr; Trapp2022).
• Figure 2: A schematic of the implementation of radial gas flows in our models. For a given inward flow velocity, $v_{r,g} < 0$, and timestsep size, $\delta t$, a given annulus, $R \rightarrow R + \delta R$, loses the gas within $R \rightarrow R - v_{r,g} \delta t$ to its "inner neighbor" at $R - \delta R \rightarrow R$. Simultaneously, this annulus gains from its "outer neighbor" following a similar prescription, though potentially with a different velocity. The fraction of the mass in each ananulus that is transferred is given by the projected area of the red ring relative to the $R \rightarrow R +\delta R$ annulus. Our analytic solutions apply the limit as $\delta R, \delta t \rightarrow 0$ to this prescription.
• Figure 3: The evolutionary history imposed upon each of our GCE models. Colored lines show the surface densities of star formation (left) and gas (middle) as functions of Galactocentric radius at lookback times of 0, 2, 4, 6, 8, and 10 Gyr, color coded according to the colorbar at the top. Dashed black lines show exponential surface density profiles with arbitrary normalizations and scale lengths of $r_\star = 2.5$ kpc (left) and $r_g = 4$ kpc (middle), broadly consistent with the observed surface density gradients Kalberla2009BlandHawthorn2016. The right panel shows the median stellar age predicted by our GCE models after radial migration is taken into account (solid red line) in comparison with our measurements from APOGEE in Johnson2025 (black stars). Summary: Each model is constrained to form radial profiles in surface density and stellar age in broad agreement with observations.
• Figure 4: Evolution in the ISM metallicity (top) and radial velocity (bottom) over time in our fiducial GCE models. From left to right: the base model with $v_{r,g} = 0$ everywhere, a constant velocity of $v_{r,g} = -1$ km/s, the GT scenario with $\dot L/L = -0.1$ Gyr$^{-1}$, the PWD scenario with $\gamma = 0.2$, and the AMD scenario with $\beta_{\phi,\text{acc}} = 0.7$ (see discussion in Section \ref{['sec:gce:scenarios']}). Solid lines show snapshots of the radial profiles color coded according to the colorbar at the top. Dotted black lines in the top row of panels show the equilibrium O abundance as a function of Galactocentric radius at the present day. Summary: Each radial gas flow scenario has a different impact on the predicted metal enrichment history, with some being more substantial than others. Inward flows tend to lead to strong variations in the equilibrium metallicity with Galactocentric radius, even if the ISM never reaches the local equilibrium abundance across much of the disk.
• Figure 5: A comparison of the ISM radial velocity (left) and metallicity profiles (right) predicted by our GCE models, marked according to the legend at the top. We shade the region corresponding to $\left|v_{r,g}\right| > 5$ km/s, which is disfavored by direct observations of galaxy velocity fields DiTeodoro2021. Black squares in the right panel mark the best-fit profile for H ii regions in the MW measured by MendezDelgado2022. Summary: Inward flows with velocities of order $\sim$$0.1 - 1$ km/s at low redshift, within allowed empirical constraints, should have significant effects on abundance growth. The ORA limit should reflect the steepest possible metallicity gradient for a given choice of GCE parameters (see discussion in Section \ref{['sec:results:dilution-steepening']}).