Galactic seismology: can a disc-crossing impulse explain the large-scale perturbations in the Milky Way's disc?

TL;DR

Gaia reveals phase-space substructures in the Milky Way disc, and this study tests whether a single disc-crossing impulse from a Sagittarius-like perturber can simultaneously generate observed spiral arms, disc corrugations, and the phase spiral. It uses a multi-physics approach with a pure N-body model and variants including inert gas and star-forming gas, comparing spiral-arm masks, corrugation profiles, the phase spiral, and the $L_{z}-\bar{V}_{R}$ wave to observations via frame transformations and feature-specific similarity maps. The results show that a single crossing can plausibly reproduce the Outer arm, Local arm, Sag-Car arm, and local corrugation, and can generate a phase spiral, but cannot reproduce all arm segments or the $L_{z}-\bar{V}_{R}$ wave; gas physics tends to weaken these matches. The findings imply that while a Sgr-like impulse likely contributed substantially to several large-scale MW disc features, fully explaining the current dynamical state of the disc requires additional triggers, such as the Galactic bar or interactions with other satellites.

Abstract

Prior to its infall, the Sagittarius (Sgr) dwarf galaxy was a major satellite with a mass of $M_{\rm sgr}\sim 10^{11}$ M$_\odot$. For the past $5-6~\mathrm{Gyr}$ , it has been heavily stripped by the Milky Way (MW), losing most of its mass while crossing the MW disc multiple times. Recent models of Milky Way disc perturbations $-$ including the spiral arms, the stellar bar, the Gaia phase spiral, and stellar and gaseous disc corrugations $-$ have identified these crossings as possible formation triggers, but have generally treated each perturbation in isolation. Here, we adopt a holistic perspective and ask whether a single disc-crossing impulse can simultaneously account for these features as observed today. We focus on simulations of single disc-crossing events by a Sgr-like perturber, and present a forensic analysis of the role of the powerful impulse in forming spiral arms, disc corrugations, the phase spiral and the `$L_{z}-\bar{V}_{R}$ wave', determined from a star's angular momentum and radial velocity, respectively. We find that a single disc crossing can reproduce reasonably well (e.g. structure, amplitude, phase) the observed local disc corrugation, and the Outer, Local and Sagittarius-Carina arm segments, implying that the last significant impulse due to a transit took place $700-1200~\mathrm{Myr}$ ago. Moreover, the $L_{z}-\bar{V}_{R}$ wave and phase spiral appear within the simulations over the same epoch and their general structure is reasonably well replicated, but not in detail. We conclude that Sgr's last significant crossing roughly a Gyr ago could be the primary cause of large-scale MW disc perturbations, but it cannot fully account for the $L_{z}-\bar{V}_{R}$ wave. Consequently, other triggers, possibly the Galactic bar or interactions with other satellites, must be considered in order to fully explain the current dynamical state of the MW's disc.

Figures (21)

• Figure 1: Illustration of 24 wedge shaped selections shaded in green used to sample the radial height profile or corrugation, overlaid on top of a synthetic galactic disc that is rotating anti-clockwise. In addition to the wedge shaped selection there are further cuts on Galactocentric radius denoted by the inner and outer dashed white circles. The selection is an approximation of the projected selection used by CorrugationYH2024 in their analysis of the local radial height profile. Note that only 24 of the 72 wedges we used for our analysis are shown here to avoid excessive visual clutter. See Sec. \ref{['sec:method_corrugation']} for a more detailed description of this selection.
• Figure 2: Spiral arm segments adapted from SpiralArmMasersReid2019, shown as they would appear if the Sun were located at $(X,Y)=(8,0)~\>\!{\rm kpc}{kpc}$ and the disc rotated anti-clockwise. Note that we do not use the four arm plus Local arm model proposed by SpiralArmMasersReid2019 which extrapolated fitted arm segments instead plotting the arm segments only across the azimuths where maser sources are observed using the parameters seen in Table 2. The innermost arms, the $3~\>\!{\rm kpc}{kpc}$(N) and Norma arms are excluded, since they exist too close to the Galactic bar which is not included in the simulations considered in this study.
• Figure 3: The arm coverage similarity of the Outer arm. The time is set relative to the time of impact and the azimuth is relative to the impact's azimuth, with positive azimuths going in the anti-clockwise direction in the direction of Galactic rotation. The impact time is outlined in black. High arm coverage is coloured dark blue whereas low arm coverage is coloured in yellow. The arm coverage is structured into diagonal bands that wrap in azimuth. The minimum time and maximum time refer to the range in time where the bands appear the most linear. A fit of the diagonal bands is also overlaid on top.
• Figure 4: Two snapshots of the density contrast at two different times. Left: The density contrast at time $T = 250~\>\!{\rm Myr}{Myr}$ where the impact has created large spanning over and underdensity regions. Right: The density contrast at a time late in the simulation where the disc has settled leading to a weakening of the spiral arms which can be seen at late times in Fig. \ref{['fig:outer_arm_similarity']}.
• Figure 5: The arm coverage similarity over azimuth and time for the Scutum-Centauri arm (top left), Sagittarius-Carina arm (top right), Local arm (bottom left) and the Perseus arm (bottom right). In each panel, the similarity traces out two separate wrapping diagonal bands corresponding to an arm and its counter arm. These bands are highly resolved some time after the impact but eventually dissipate as the disc settles, at varying rates across arms.