Asymmetric Mean Metallicity Distribution of the Milky Way’s Disk

I present the mean metallicity distribution of stars in the Milky Way based on photometry from the Sloan Digital Sky Survey. I utilize an empirically calibrated set of stellar isochrones developed in previous work to estimate the metallicities of individual stars to a precision of 0.2 dex for reasonably bright stars across the survey area. I also obtain more precise metallicity estimates using priors from the Gaia parallaxes for relatively nearby stars. Close to the Galactic mid-plane ($| Z| \lt 2$ kpc), a mean metallicity map reveals deviations from the mirror symmetry between the northern and southern hemispheres, displaying wave-like oscillations. The observed metallicity asymmetry structure is almost parallel to the Galactic mid-plane, and coincides with the previously known asymmetry in the stellar number density distribution. This result reinforces the previous notion of the plane-parallel vertical waves propagating through the disk, in which a local metallicity perturbation from the mean vertical metallicity gradient is induced by the phase-space wrapping of stars in the Z–VZ plane. The maximum amplitude of the metallicity asymmetry (Δ[Fe/H] ∼ 0.05) implies that these stars have been pulled away from the Galactic mid-plane by an order of ${\rm{\Delta }}| Z| \sim 80$ pc as a massive halo substructure such as the Sagittarius dwarf galaxy plunged through the Milky Way. This work provides evidence that the Gaia phase-space spiral may continue out to $| Z| \sim 1.5\,\mathrm{kpc}$.