Chemical dissection of merger-induced $m=1$ lopsidedness in Milky Way-like galaxies

TL;DR

This study uses high-resolution simulations to explore how minor mergers induce m=1 lopsidedness in Milky Way-like galaxies, revealing that metal-rich stars exhibit stronger asymmetries and are more affected by such interactions.

Contribution

It demonstrates that merger-driven lopsidedness is more pronounced in metal-rich stellar populations and links chemical composition to dynamical asymmetries in galaxy evolution.

Findings

Metal-rich stars show stronger m=1 lopsidedness in density and velocity.

Minor mergers cause transient disc-halo offsets, especially in metal-rich populations.

Lopsidedness strength correlates with metallicity in observed galaxies.

Abstract

The Milky Way harbours a prominent m=1 lopsided distortion in both stellar and neutral gas distributions. On the other hand, chemo-dynamical studies have been proven to be effective in grasping the overall evolution of galaxies. Here, we investigate systematically the excitation and evolution of a merger-driven $m=1$ lopsidedness in a Milky Way (MW)-like host galaxy, as a function of chemical distribution of stars. Using seven dissipationless, high-resolution $N$-body simulations of minor mergers (between a MW-like host and a satellite) under varying orbital configurations (prograde/retrograde and different orientation of the satellite orbital plane), we first show that a tidal interaction excites a prominent $m=1$ lopsidedness in the stellar density and velocity distribution of the MW-like host. Assigning, a posteriori, metallicities to stellar particles of the MW-like host based on the current observational constraints, we sub-divide the stars into metal-rich ([Fe/H] > 0), metal-intermediate (-0.5 < [Fe/H] <0), and metal-poor (- 0.5 < [Fe/H]) populations. We demonstrate that metal-rich population always show a much stronger $m=1$ lopsidedness in both density and velocity distributions when compared to other two populations. This trend holds true for all minor merger model considered here, regardless of their orbital configurations. Furthermore, minor merger also triggers a transient off-centred stellar disc-dark matter halo configuration, with metal-rich population showing the highest degree of disc-halo offset. We show that the metal-rich population which is kinematically colder (i.e. lower velocity dispersion) by construction, is more susceptible to external perturbations. Lastly, using a catalogue of photometry and metalicity for the LMC, we show that the strength of the $m=1$ distortion (predominantly in the form of an one-arm spiral) in stars increases with metallicity as well.