The Galactic Disk North-south Asymmetry in Metallicity May Be A New Tracer for the Disk Warp

TL;DR

This study demonstrates that the north-south asymmetry in stellar metallicity can serve as a new chemical tracer for the Galactic disk warp, complementing kinematic and stellar distribution methods.

Contribution

It introduces the use of stellar metallicity asymmetry as a novel tracer for the Galactic disk warp, supported by analysis of a large RC star sample.

Findings

Metallicity asymmetry correlates with disk warp morphology.

The warp line of node is at approximately 4.24 degrees.

The metallicity-based warp model aligns with Cepheid observations.

Abstract

Galactic disk warp has been widely characterized by stellar distributions and stellar kinematics but has not been traced by stellar chemistry. Here, we use a sample with over 170,000 red clump (RC) stars selected from LAMOST and APOGEE first to establish a correlation between the north-south asymmetry in metallicity ([Fe/H]) and the disk warp. Our results indicate that the height of the [Fe/H] mid-plane for the whole RC sample stars is accurately described as $Z_{w}$ = 0.017 ($R$ $-$ 7.112)$^{2}$ sin($\phi$ $-$ 9.218). This morphology aligns closely with the warp traced by Cepheids, suggesting that the disk north-south asymmetry in [Fe/H] may serve as a new tracer for the Galactic warp. Our detailed analysis of the young/thin disk stars of this RC sample suggests that its warp is well-modeled as $Z_{w}$ = 0.016 ($R$ $-$ 6.507)$^{2}$ sin($\phi$ $-$ 4.240), indicating that the line of node (LON) of the Galactic warp is oriented at 4.240$_{-1.747}^{+1.641}$ degree.