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

TL;DR

This study investigates how minor mergers induce $m=1$ lopsidedness in Milky Way–like galaxies and whether the response depends on stellar metallicity. Using seven high-resolution, dissipationless $N$-body simulations with varied orbital configurations, the authors assign metallicities a posteriori to partition stars into metal-rich, metal-intermediate, and metal-poor groups and quantify density and kinematic $m=1$ via the Fourier amplitude $A_1/A_0$ for both the density and the mean azimuthal velocity $ig<V_ extphiig>$. They find metal-rich, dynamically colder stars exhibit consistently stronger $m=1$ distortions in both density and velocity fields, a trend robust across merger configurations, and show a transient disc–DM halo off-set linked to the tidal interaction. Extending the analysis to the LMC with Gaia-based metallicities reveals a similar metallicity-dependent enhancement of the $m=1$ distortion, particularly for metal-rich stars, though interpretation is nuanced by the LMC–SMC interaction. Overall, the work demonstrates a strong metallicity–kinematics coupling as a driver of merger-induced lopsidedness and its secular evolution in MW-like systems.

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.

Figures (11)

• Figure 1: Metallicity distribution function (MDF) adopted for this work, at $t=0$ for the fiducial minor merger model MWsat_n1_$\Phi 30$. The thin disc is associated with the metal-rich ($\rm [Fe/H]$$>0$), the intermediate with the metal-intermediate ($-0.5 \leq \hbox{$\rm [Fe/H]$} <0$), and the thick disc with the metal-poor ($\hbox{$\rm [Fe/H]$} < -0.5$) population. For further details, see the text.
• Figure 2: Chemical dissection of $m=1$ lopsidedness: Face-on ($x-y$-plane) stellar density distribution (MW-like host+satellite), for the metal-rich (top row), metal-intermediate (middle row), and metal-poor (bottom row) populations, at different times capturing different pericentre passages and the merger epoch of the satellite, for the model MWsat_n1_$\Phi 30$. A prominent $m=1$ lopsided pattern is excited in the stellar density distribution of MW-like host after each of the pericentre passages.
• Figure 3: Temporal evolution of the $m=1$ Fourier amplitude for the stellar density (averaged between $1-5 \ R_{\rm d, thin}$), calculated separately for the metal-rich, metal-intermediate, and metal-poor populations (see the legend) using Eq. \ref{['eq:avg_fourier_density']}, for all minor merger models considered here. Vertical magenta dashed-dotted lines denote the epochs of pericentre passages while vertical black dashed-dotted line denotes the epoch of merger. Here, $R_{\rm d, thin} = 4.8 \hbox{$\>{\rm kpc}$}$. The metal-rich population shows a higher degree of lopsidedness when compared to other two populations.
• Figure 4: Chemical dissection of the $m=1$ kinematic lopsidedness: Face-on ($x-y$-plane) distribution of the mean azimuthal velocity, $\left<{V_{\phi}}\right>$ (MW-like host only), for the metal-rich (top row), metal-intermediate (middle row), and metal-poor (bottom row) populations, at different times capturing different pericentre passages and the merger epoch of the satellite, for the model MWsat_n1_$\Phi 30$. The black dotted lines denote the orientation of the kinematic lopsidedness, i.e., the $\varphi_1 (\hbox{$\left<{V_{\phi}}\right>$})$ values (using Eq. \ref{['eq:fourier_calc_kineLop']}) averaged within $1-5 \ R_{\rm d,thin}$. Each of the pericentre passages excites an asymmetry in the $\left<{V_{\phi}}\right>$ distribution.
• Figure 5: Temporal evolution of the $m=1$ Fourier amplitude for the stellar mean azimuthal velocity $\hbox{$\left<{V_{\phi}}\right>$}$ (averaged between $1-5 \ R_{\rm d, thin}$), calculated separately for the metal-rich, metal-intermediate, and metal-poor populations (see the legend), for 5 minor merger models considered here. Vertical magenta dashed-dotted lines denote the epochs of pericentre passages while vertical black dashed-dotted line denotes the epoch of merger. Here, $R_{\rm d, thin} = 4.8 \hbox{$\>{\rm kpc}$}$. The metal-rich populations shows a higher degree of kinematic lopsidedness when compared to other two populations.