# Asymmetric Mean Metallicity Distribution of the Milky Way's Disk

## TL;DR

This study maps the Milky Way's stellar metallicity distribution using SDSS and Gaia data, revealing asymmetries and wave-like oscillations linked to vertical disk perturbations and halo interactions.

## Contribution

It introduces a detailed metallicity map of the Milky Way's disk, highlighting asymmetries and their connection to vertical waves and halo substructures.

## Key findings

- Detected metallicity asymmetry of ~0.05 dex near the mid-plane.
- Identified wave-like oscillations aligned with the Galactic plane.
- Linked metallicity perturbations to halo interactions like Sagittarius dwarf galaxy.

## Abstract

I present the mean metallicity distribution of stars in the Milky Way Galaxy 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 ($|Z|<2$ 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 $Z$-$V_Z$ plane. The maximum amplitude of the metallicity asymmetry ($\Delta$[Fe/H]$\sim0.05$) implies that these stars have been pulled away from the Galactic mid-plane by an order of $\Delta|Z|\sim80$ 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 $|Z|\sim1.5$ kpc.