Asymmetric Mean Metallicity Distribution of the Milky Way's Disk

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]$\sim 0.05$) implies that these stars have been pulled away from the Galactic mid-plane by an order of $\Delta |Z|\sim 80$ 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 $|Z|\sim 1.5$ kpc.

Accepted for publication in the Astrophysical Journal Letters

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• The Milky Way, Coming into Focus: Precision Astrometry Probes its Evolution, and its Dark Matter
• A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems
• Investigating the amplitude and rotation of the phase spiral in the Milky Way outer disc
• Applying Noether's theorem to matter in the Milky Way: evidence for external perturbations and non-steady-state effects from Gaia Data Release 2
• The Vertical Structure and Asymmetry of Mg ii-enriched Gas in the Milky Way Disk