The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars are Metal-Rich

• Main Authors: Johnson, James W, Weinberg, David H, Blanc, Guillermo A, Bonaca, Ana, Rudie, Gwen C, Yuxi, Lu, Chu, Bronwyn Reichardt, Griffith, Emily J, Sit, Tawny, Johnson, Jennifer A, Dubay, Liam O, Weller, Miqaela K, Boyea, Daniel A, Bird, Jonathan C
• Format: Journal Article
• Language: English
• Published: 17-10-2024
• Subjects: Physics - Astrophysics of Galaxies; Physics - Cosmology and Nongalactic Astrophysics
• DOI: 10.48550/arxiv.2410.13256

Metallicities of both gas and stars decline toward large radii in spiral
galaxies, a trend known as the radial metallicity gradient. We quantify the
evolution of the metallicity gradient in the Milky Way as traced by APOGEE red
giants with age estimates from machine learning algorithms. Stars up to ages of
$\sim$9 Gyr follow a similar relation between metallicity and Galactocentric
radius. This constancy challenges current models of Galactic chemical
evolution, which typically predict lower metallicities for older stellar
populations. Our results favor an equilibrium scenario, in which the gas-phase
gradient reaches a nearly constant normalization early in the disk lifetime.
Using a fiducial choice of parameters, we demonstrate that one possible origin
of this behavior is an outflow that more readily ejects gas from the
interstellar medium with increasing Galactocentric radius. A direct effect of
the outflow is that baryons do not remain in the interstellar medium for long,
which causes the ratio of star formation to accretion, $\dot{\Sigma}_\star /
\dot{\Sigma}_\text{in}$, to quickly become constant. This ratio is closely
related to the local equilibrium metallicity, since its numerator and
denominator set the rates of metal production by stars and hydrogen gained
through accretion, respectively. Building in a merger event results in a
perturbation that evolves back toward the equilibrium state on $\sim$Gyr
timescales. Under the equilibrium scenario, the radial metallicity gradient is
not a consequence of the inside-out growth of the disk but instead reflects a
trend of declining $\dot{\Sigma}_\star / \dot{\Sigma}_\text{in}$ with
increasing Galactocentric radius.