Stellar metallicity variations across spiral arms in disk galaxies with multiple populations

TL;DR

This study uses high-resolution N-body simulations to show that azimuthal metallicity variations in spiral galaxy disks can naturally occur without initial radial metallicity gradients, due to differential responses of stellar populations to spiral arms.

Contribution

It demonstrates that metallicity variations across spiral arms can arise from kinematic differences in stellar populations, independent of initial radial metallicity gradients.

Findings

Cooler populations contribute more to spiral arms.

Azimuthal metallicity variations occur without initial radial gradients.

Variations are linked to kinematic-metallicity relation.

Abstract

This letter studies the formation of azimuthal metallicity variations in the disks of spiral galaxies in the absence of initial radial metallicity gradients. Using high-resolution $N$-body simulations, we model composite stellar discs, made of kinematically cold and hot stellar populations, and study their response to spiral arm perturbations. We find that, as expected, disk populations with different kinematics respond differently to a spiral perturbation, with the tendency for dynamically cooler populations to show a larger fractional contribution to spiral arms than dynamically hotter populations. By assuming a relation between kinematics and metallicity, namely the hotter the population, the more metal-poor it is, this differential response to the spiral arm perturbations naturally leads to azimuthal variations in the mean metallicity of stars in the simulated disk. Thus, azimuthal variations in the mean metallicity of stars across a spiral galaxy are not necessarily a consequence of the reshaping, by radial migration, of an initial radial metallicity gradient. They indeed arise naturally also in stellar disks which have initially only a negative vertical metallicity gradient.