Asymmetric Mean Metallicity Distribution of the Milky Way's Disk

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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 (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 (Δ[Fe/H] ∼ 0.05) implies that these stars have been pulled away from the Galactic mid-plane by an order of 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| ∼ 1.5 kpc.

Original languageEnglish
Article numberL31
JournalAstrophysical Journal Letters
Issue number2
StatePublished - 20 Jun 2019

Bibliographical note

Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved.


  • Galaxy: abundances
  • Galaxy: disk
  • Galaxy: structure
  • stars: abundances


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