Azimuthal Misalignments in Stellar Warp Structure as Dynamical Tracers of Mergers in Milky Way-like Galaxies

TL;DR

The paper tests whether major mergers leave a lasting azimuthal misalignment between warp structures traced by stars of different ages in Milky Way–like galaxies. Using high-resolution IllustrisTNG50 MW analogs, it identifies post-merger warps aligned with the gas disk and pre-merger warps that are oriented differently, with the misalignment quantified via LOESS fits to ⟨$Z$⟩(Φ) and a phase offset ΔΦ. After correcting for differential rotation in a corotating frame, the misalignment remains for several models, implying a genuine dynamical imprint of the merger rather than kinematic artifacts. These results support a merger-driven origin for galactic warps and offer a new avenue to constrain the timing of past mergers, including the Gaia-Sausage-Enceladus event in the Milky Way, by examining age-dependent warp phase offsets; observational prospects with Gaia, 4MOST, and SDSS-V are discussed, along with caveats about age dating and alternative warp mechanisms.

Abstract

We investigate the origin of warps in stellar disks using high-resolution Milky Way analogs from the IllustrisTNG50 simulation. Focusing on galaxies that experienced a major merger, we identify a characteristic azimuthal misalignment between the warp structures of stellar populations formed before and after the merger. This misalignment persists even after correcting for differential rotation, suggesting it is a dynamical imprint of the merger rather than a consequence of internal kinematics. In contrast, galaxies without significant merger events show no such offset between stellar populations of different ages. These findings support the scenario in which mergers can induce long-lived warps and leave detectable structural signatures in stellar disks. Applied to the Milky Way, this approach offers a potential way to test whether the Gaia-Sausage-Enceladus merger contributed to the formation of the Galactic warp. It may also provide an independent means to constrain the timing of such merger events by examining the phase offsets in the stellar warp as a function of stellar age.

Figures (4)

• Figure 1: Density projection of warped Milky Way analogs from IllustrisTNG50 in the X-Y and X-Z plane.
• Figure 2: X-Y plane maps of average vertical displacement $\langle Z \rangle$ for present-day snapshots (t=0) of Subhalo IDs 540920, 482889, and 476266. These visualizations contrast the warp signatures found within the gaseous disk, stars formed after the merger, and stars formed before the merger. Each map illustrates how the galaxy's vertical structure varies among its distinct stellar populations and gas, shedding light on the influence of the merger event on the observed galactic warp. In each model, the warp in the post-merger stellar disk aligns with the gaseous warp. In contrast, the pre-merger stellar population exhibits a distinct warp signature, appearing at a different orientation.
• Figure 3: Azimuthal variation of the average vertical displacement $\langle Z \rangle$ for the present-day snapshot (t=0) of each model galaxy. Top panel: For each galaxy that developed a warp due to a major merger, we analyze the stars contributing to the warp, separating them into populations formed before and after the merger. This plot reveals the differences in warp structure between these two stellar populations, highlighting an intriguing azimuthal misalignment in the warp signature between pre- and post-merger stars. Bottom panel: Same as the top panel, except that we corrected for the misalignment due to the differential rotation for pre-merger and post-merger populations.
• Figure 4: Azimuthal variation of the average vertical displacement $\langle Z \rangle$ for the present-day snapshot ($t = 0$) of the model galaxy 459557. As this galaxy does not undergo a significant merger, the stellar population is divided into three age bins 0--4 Gyr, 4--8 Gyr, and $>$8 Gyr to compare their respective warp structures. (a) Maps of $\langle Z \rangle$ in the X--Y plane for the gaseous disk and stars in each age bin at the present day (b) Azimuthal profiles of $\langle Z \rangle$ for stars in each age bin, illustrating the similarities in the warp signature across stellar populations.