A North-South Metallicity Asymmetry in the Outer Galactic disk -- Evidence for the Pericentric Passage of the Sagittarius Dwarf Galaxy

TL;DR

This study maps the metallicity distribution in the outer Galactic disk, revealing a North-South asymmetry likely caused by the Sagittarius dwarf galaxy's recent pericentric passage, affecting stars of all ages.

Contribution

It provides the first detailed metallicity asymmetry map in the outer disk and links it to the Sagittarius dwarf galaxy's tidal influence.

Findings

North-South metallicity asymmetry exists in the outer disk.

Asymmetry is present across all stellar ages, but less in older populations.

The asymmetry is likely due to the Sagittarius dwarf galaxy's recent pericentric passage.

Abstract

We present maps of the mean metallicity distributions on the Galactocentric $R$--$Z$ plane at different azimuthal angles using red clump stars selected from the LAMOST and APOGEE surveys. In the inner disk ($R < $ 11\,kpc), the metallicity distribution is symmetric between the upper and lower disk. However, we find a North-South metallicity asymmetry in the outer disk ($R > 11$\,kpc), especially towards the anti-Galactic center ($-5^\circ < \Phi < 15^\circ$) direction. By further dissecting the map in age space, we detect this asymmetry across all mono-age stellar populations. However, the asymmetry is less pronounced in older populations ($\tau > 8$ Gyr) compared to younger ones ($\tau < 6$\,Gyr). This reduced significance likely stems from three factors: larger age uncertainties, fewer stars in the outer disk, and the kinematically hotter nature of older populations. The observed metallicity asymmetry may be the consequence of the purturbation of the recent pericentric passage through the Galactic disk and tidal force of the well-known Sagittarius dwarf galaxy.