A Low-mass Model of The Milky Way: The Disk Warp Resulting from A Galaxy Merger

TL;DR

The paper investigates how the Milky Way's disk warp can arise from a gas-rich Gaia-Sausage-Enceladus merger within a low-mass MW model. It builds a GSE-merger framework with Einasto halos and runs GIZMO-based, idealized simulations across a range of orbital inclinations to study warp evolution and precession. The key finding is that warp is driven by the asymmetric DM halo and exhibits two dynamical regimes: a short-term angular-momentum exchange (seesaw) and a secular alignment that damps both halo tilt and warp, with high-inclination mergers showing persistent prograde precession but not matching the MW’s observed strength. The results underscore the dominance of inner-halo gravitational torques in warp generation, the regeneration nature of warps, and the potential need for additional perturbers (e.g., Sgr, LMC) to fully explain the Milky Way's warp history and kinematics.

Abstract

Previous studies have shown that disk warps can result from galaxy mergers. Recent research indicates a noticeable decline in the rotation curve (RC) of the Milky Way (MW), suggesting the need for a new low-mass model to describe its dynamical features. This study constructs a new Gaia-Sausage-Enceladus (GSE) merger model to characterize the RC features of our galaxy. We use the GIZMO code to simulate mergers with various orbital parameters to investigate how the disk warp evolves under different conditions. This simulation demonstrates the evolutionary mechanism of disk warp, which arises due to the asymmetric gravitational potential of the dark matter (DM) halo generated universally by galaxy mergers. The results indicate that the tilt angle of the DM halo partly reflects the gravitational strength at the $Z=0$ plane, while the gravitational strength on the disk plane reflects the amplitude of disk warp. We identify a dual-regime interaction mechanism driven by the asymmetric halo potential. On short timescales, we find a distinct anti-correlation between the halo's tilt angle and the disk's warp amplitude, indicating a `seesaw' mechanism of angular momentum exchange. On secular timescales, however, dynamical friction drives a global alignment, causing both the halo tilt and the warp amplitude to decay simultaneously. Furthermore, we demonstrate that high-inclination mergers can sustain long-lived prograde precession, where the persistent yet decaying gravitational torque maintains the prograde bending mode against differential wind-up.

Figures (11)

• Figure 1: Rotation curve decomposition of the simulated galaxy at $t=11$ Gyr (for the $15^{\circ}$ inclination model). The solid lines represent the circular velocity contributions from different galactic components (DM, Gas, Stars and total), calculated using the PROFILE function of the PYNBODY library Pontzen. The data points with error bars indicate observational constraints from the Milky Way, taken from Jiao (blue triangles) and Ou (red stars).
• Figure 2: $Top\enspace left\enspace panel$ shows the DM density profile along with the radius. $Top\enspace middle\enspace panel$ shows mass enclosed within a given radius of DM halo. It can be observed that the mass of the dark matter halo in our model is concentrated within $\mathrm{0.2\,r_{vir}}$. $Top\enspace right\enspace panel$ is the anisotropy parameter $\beta$ of the stellar halo (blue solid line) and the DM halo (gray solid line) with Galactocentric distance in a spherical coordinate system. $Bottom\enspace left\enspace panel$ shows the triaxiality of the DM halo, which is measured with DM particles for each bins, as the radius is divided into 100 bins with the same number of particles from 0 to $\mathrm{r_{vir}}$, the dense polylines around $\sim \mathrm{0.1\,r_{vir}}$ exhibits a highly oblate shape. $Bottom\enspace middle\enspace panel$ is the tilt angle between DM halo's short axis with the disk plane in each radius bins, and it shows an increasing trend from the center outward. $Bottom\enspace right\enspace panel$ shows hist profile of DM halo energy distribution within $\mathrm{r_{vir}}$.
• Figure 3: Time evolution of the DM halo's shape and orientation for different inclination groups (indicated by different colors and lines in each subgraph), calculated using DM particles within the central $20$ kpc . $Top\enspace panels$ show the evolution of the DM halo's triaxiality parameter, defined as $T=(1-p^{2})/(1-q^{2})$, from 3 to 11 Gyr. The horizontal dashed line marks $T=0.3$; values below this threshold indicate an oblate shape, while values $T\geq0.6$ indicate a prolate shape. $Bottom\enspace panels$ indicate the evolution of the DM halo's tilted angle with respect to the disk panel, derived by tracking the short axis of the DM halo. The halo evolution shows distinct behaviors roughly based on inclination: high-inclination models ($i>45^{\circ}$) , and low-inclination models ($i\leq45^{\circ}$), they both retain tilted with respect to the disk all the time.
• Figure 4: $Top\enspace panels$ show the evolution of stellar warp and the $Bottom\enspace panels$ indicate the gas disk warp amplitude, both measured at R = 16 kpc, from 3 Gyr to 11 Gyr, and the stellar disk amplitude is fitting with stars younger than 1.5 Gyr. The curves represent different inclination models as defined in Figure \ref{['F3']}. The dash lines indicate the amplitude of MW in observation. High-inclination models show larger initial amplitudes, and the warp displays a regeneration phenomenon: the amplitude often decays to a near-flat state before rising again to a new peak.
• Figure 5: Evolution of stellar disk warp precession $\omega$ of the higher and lower inclination groups from $\mathrm{3\,Gyr}$ to $\mathrm{11\,Gyr}$ as measured with young stars within the radial range of $\mathrm{R = 7-17\,kpc}$, which are divided into 20 bins at intervals of $\mathrm{0.5\,kpc}$. The color means the direction of precession, while red means prograde ($\omega\,\textgreater\,0$), blue means retrograde ($\omega\,\textless\,0$) and the white color indicates the non-precession state. Some bins are colored in gray to represent the non-warp region where $R < R_{w}$. Here, $R_{w}$ is the onset radius of the warp. The kinematic behavior differs by inclination: high-inclination models frequently exhibit long-term prograde precession, while low-inclination models are dominated by retrograde precession with only transient high prograde features.