Kinematic Signatures in the Stellar Halo from Cosmological Encounters between the Milky Way and its Clouds

TL;DR

This work addresses how a massive satellite such as the Large Magellanic Cloud (LMC) imprints kinematic signatures on the Milky Way's stellar halo. It analyzes five MW-mass halos from the FIRE-2 Latte cosmological zoom-in simulations with an LMC-like infall, removes LMC-analog contamination, and applies spherical-harmonic decomposition to isolate global reflex motion (dipole, $\ell=1$) and the local dynamical-friction wake (quadrupole, $\ell=2$). The study finds that the global dipole dominates radial and vertical velocities, while a local wake produces a weaker quadrupole in the azimuthal velocity near infall; the strength and orientation depend on the pericenter mass ratio $\mathrm{PMR}_{\mathrm{peri}}$ and pericenter distance $r_{\mathrm{peri}}$, and these signatures persist after LMC removal. This framework, with its mode-based diagnostics, has direct implications for detecting MW-LMC interactions in upcoming surveys and guiding observational strategies using tracers like M giants and RR Lyrae, supported by mock catalogs from tools such as $\text{py-ananke}$ for realistic sky maps.

Abstract

Recent theoretical and observational analysis of the interaction between the Milky Way (MW) and LMC suggest that it has a significant dynamical impact on the MW's stellar halo. We investigate this effect using simulations from the Latte project, a simulation suite from the Feedback In Realistic Environments 2 (FIRE-2) Project. By comparing simulations with and without an LMC-analog interaction, we show that fully cosmological LMC interactions create prominent velocity asymmetry in the stellar halo of the MW, resulting from both barycentric displacement (the "reflex motion") and the dynamical wake of the LMC. The strength and direction of this asymmetry depend on the mass ratio at pericenter and orbit of the LMC analog. We perform a spherical-harmonic decomposition of the velocities of halo star particles to confirm that the identified signatures are LMC-induced and persist even when LMC star particles are removed. We also show that this strategy separates and individually detects the dipole (l=1) of the global reflex motion and the quadrupole (l=2) of the local wake. These asymmetries are consistent with those identified in previous work using non-cosmological simulations; the dipole is easily distinguishable from other complex halo substructure using spherical harmonics while the quadrupole is sometimes confused. These findings support the detectability of MW--LMC interaction signatures in upcoming observational surveys of the MW stellar halo.

Figures (8)

• Figure 1: Formation distance vs time of formation for star particles in simulation m12i (top left), simulation m12b (top right), the isolated m12b LMC analog (bottom left), and m12b without the LMC analog (bottom right). Individual satellite galaxies appear as thin lines, and the host galaxy's disk appears as a dense bar below 25 kpc. We define 8.81 Gyr as present day because of the comparable location and trajectory of the LMC analog and current LMC. In all analysis of simulations with an LMC analog, we use the selection with the LMC analog removed.
• Figure 2: Mass weighted distribution of stars particles in m12b with (left) and without (center) the LMC analog and in m12i (right) simulated galaxies. Galaxies are face on in the xy plane. In m12b before removal, the LMC analog can be identified as the smaller galaxy $\sim40$ kpc from the center. The LMC analog's infall trajectory is shown in all m12b plots as an orange line.
• Figure 3: Face-on views of cylindrical velocities for the stellar halo of host galaxies of m12w, m12c, m12b, m12f, and m12i simulations. In simulations with an LMC analog, the most notable infall signature is a global kinematic asymmetry. This asymmetry is most prominent in the radial velocity signature (left column). We include all stars within a vertical slab of 100kpc above and below the disk plane to capture features of the stellar halo. Star particles belonging to the LMC analog are removed in our analysis, and the LMC analog's trajectory is displayed as a pink line.
• Figure 4: Power spectra for mean cylindrical velocity signatures in simulations m12b, m12w, m12f, m12c, and m12i (rows 1--5 respectively). In all simulations with an LMC analog, the global reflex motion is evident as a high $\ell = 1$ power in at least the radial velocity signature (left column), though for most simulations the strong power persists through the other signatures as well. Dashed lines reflect the power spectra with the LMC analog included, whereas solid lines have the LMC analog removed. Spectra are computed for spherical shells at increasing radii, starting from the infall region and extending outward in 10 kpc increments to 40 kpc, followed by 40--60 kpc and 60--100 kpc shells. We highlight the differing $y$-axis ranges for each simulation, which reflect differing powers based on the simulation's pericenter mass ratio ($\mathrm{PMR}_{\mathrm{peri}}$) and radius ($r_{\mathrm{peri}})$
• Figure 5: Top row: Average velocity maps for cylindrical velocity $v_R$ (left), $v_\phi$ (center), and $v_z$ (right) of m12b stars located 30--40 kpc from the galactic center. The angular trajectory of the LMC analog is marked in red, with its present-day location marked by an X. Center row: Spherical harmonic expansion for the average velocity maps above. The maximum expansion term is set at $\ell_{\mathrm{max}} = 5$ to highlight low order features. Bottom row: Magnitude of $a_{\ell m}$ coefficients in the spherical harmonic expansion. Different colors correspond to degrees of $m$. In both radial and vertical velocity, the dominant mode is $\ell=1$, capturing the dynamic asymmetries that arise due to the global reflex motion. The azimuthal velocity is dominated by the $\ell = 2$, $m= \pm 2$ mode, which captures a local, outward motion resulting from the local dynamical friction wake.