Shaping the Milky Way. II. The dark matter halo's response to the LMC's passage in a cosmological context

TL;DR

This paper studies how Milky Way–mass halos respond to LMC-like perturbations in a cosmological context using Basis Function Expansions (BFE). By applying BFE to 18 MW-LMC analogs from the MWest suite and comparing with 8 quiescent Symphony halos, the authors decompose the halo response into monopole, dipole, and quadrupole components, linking the dipole to COM displacement and the quadrupole to halo triaxiality and the dynamical-friction wake. They find that the dipole amplitude scales with the square of the MW–LMC mass ratio and peaks 0.2–0.7 Gyr after pericenter, while the quadrupole strength is governed by the pre-infall halo shape and the wake, peaking near pericenter. The study demonstrates how joint dipole and quadrupole measurements can disentangle the LMC’s perturbations from the MW’s initial halo structure, offering a pathway to recover the MW’s pre-infall halo and to constrain the LMC’s mass using upcoming wide-field surveys.

Abstract

The distribution of dark matter in the Milky Way (MW) is expected to exhibit a large-scale dynamical response to the recent infall of the LMC. This event produces a dynamical friction wake and shifts the MW's halo density center. The structure of this response encodes information about the LMC- MW mass ratio, the LMC's orbit, the MW halo's pre-infall structure and could provide constraints on dark matter physics. To extract this information, a method to separate these effects and recover the initial shape of the MW's halo is required. Here, we use basis function expansions to analyze the halo response in eighteen simulations of MW-LMC-like interactions from the MWest cosmological, dark-matter-only zoom-in simulations. The results show that mergers similar to the LMC consistently generate a significant dipole and a secondary quadrupole response in the halo. The dipole arises from the host density center displacement and halo distortions, and its amplitude scales as the square of the MW-LMC mass ratio, peaking 0.2-0.7 Gyr after the LMC's pericenter. The quadrupole's strength depends primarily on the original axis ratios of the host halo, though contributions from the dynamical friction wake cause it to peak less than 0.3 Gyr before pericenter. Future measurements of both the dipole and quadrupole imprints of the LMC's passage in the density of the MW's stellar halo should be able to disentangle these effects and provide insight into the initial structure of the MW's halo, the MW's response, and the mass of the LMC.

Figures (15)

• Figure 1: Left: Orbit of the LMC analogs for 18 MWest halos included in this analysis are colored by the merger ratio ($M_{\rm LMC}/M_{\rm MW}$). For two of the hosts (229 and 659), the LMC is on its second pericentric passage, having had an earlier pericenter at $r > 100$ kpc in the halo, while for another host, the LMC has not yet reached pericenter by the final snapshot. We have normalized all the times to the pericenter passage of the LMC analog. Middle: Histogram of merger ratios between $M_{\rm LMC}/M_{\rm MW}$ for the 18 halos in the MWest suite. The merger ratios range from a ratio of 1:100 at the low end to 1:2 at the high end, with a median ratio of 1:6. Right: Distribution of pericentric distances of the LMC analogs. For comparison, the derived pericentric distance for the LMC is shown with the black vertical line.
• Figure 2: Left: Percentage error on the gravitational potential reconstructed using the BFE compared to the potential computed with the tree code method for the raw particle data. The errors across the $z=0$ kpc slice are within 2.5%. Right: Spherically averaged density residuals between the NFW basis and the particle data of all the 18 MWest halos at infall (center panel) and at pericenter (right panel). Beyond 10 kpc, the residuals are within $\approx 5\%$. In the inner halos, the density computed from the particles are subject to Poisson noise, leading to larger residuals.
• Figure 3: Example density field computation using the BFE and decomposition for Halo 349 from the MWest simulation suite, showing from left to right (1) projected density of smoothly accreted host particles, (2) full BFE with $l=5$ and $n=10$, (3) density of $l=1$ harmonic relative to $l=0$, (4) density of $l=2$ harmonic relative to $l=0$, and (5) density of $l>2$ harmonics relative to $l=0$. For the leftmost plot, the particle density is projected in the X-Y plane. The plot shows a cross-sectional slice at $z=0$. The colorbars for panels (1) and (2) are scaled arbitrarily, while plots (3-5) show the density contrast. The dashed line shows the orbit of the LMC analog, and the black dot shows the current position (chosen to be roughly at the pericentric passage).
• Figure 4: Left: Mean gravitational power relative to the monpole ($W_0$) through the evolution of each halo for the $l=1$--5 modes. On average, the quadrupole ($l=2$) and dipole ($l=1$) harmonics are the second and third most dominant modes after the monopole.
• Figure 5: Mean gravitational power averaged over the MWest suite for each harmonic mode. Error bars show the standard deviation across all halos over their evolution. DM halos are mainly characterized by the monopole, quadrupole, and dipole terms. Higher-order modes have lower amplitude and may not affect the halo as a whole, but they contain information about smaller-scale perturbations.