Galactic seismology: joint evolution of impact-triggered stellar and gaseous disc corrugations

Abstract

Evidence for wave-like corrugations are well established in the Milky Way and in nearby disc galaxies. These were originally detected as a displacement of the interstellar medium about the midplane, either in terms of vertical distance or vertical velocity. Over the past decade, similar patterns have emerged in the Milky Way’s stellar disc. We investigate how these vertical waves are triggered by a passing satellite. Using high-resolution N-body/hydrodynamical simulations, we systematically study how the corrugations set up and evolve jointly in the stellar and gaseous discs. We find that the gas corrugations follow the stellar corrugations, i.e. they are initially in phase although, after a few rotation periods (500-700 Myr), the distinct waves separate and thereafter evolve in different ways. The spatial and kinematic amplitudes (and thus the energy) of the corrugations dampen with time, with the gaseous corrugation settling at a faster rate (∼800 Myr versus ∼1 Gyr). In contrast, the vertical energy of individual disc stars is fairly constant throughout the galaxy’s evolution. This difference arises because corrugations are an emergent phenomenon supported by the collective, ordered motions of co-spatial ensembles of stars. We show that the damping of the stellar corrugations can be understood as a consequence of incomplete phase mixing, while the damping of the gaseous corrugations is a natural consequence of the dissipative nature of the gas. We suggest that – in the absence of further, strong perturbations – the degree of correlation between the stellar and gaseous waves may help to age-date the phenomenon.

A new resonance-like feature in the outer disc of the Milky Way

“…The setup of the collisionless components is identical to the model discussed in Tepper-Garcia et al (2021). The setup of the gas disc follows (Tepper-García et al 2022). More details about this model will be provided elsewhere (Tepper-García et al, in prep.…”

“…However, the resonances must then set up on a very short timescale, that is, in less than one rotation period, and this is not easy to do. Furthermore, it has long been known that the gas disc is corrugated with a wave amplitude of roughly 300 pc (see references in Tepper-García et al 2022). The phase-spiral effect is dominated by the younger stars (Bland-Hawthorn et al 2019).…”

Topography of the Young Galactic Disk: Spatial and Kinematic Patterns of Clustered Star Formation in the Solar Neighborhood

“…The physical mechanisms proposed in the literature for injecting these enormous amounts of momentum and energy into the solar neighborhood have changed over time (see, e.g., Alfaro & Efremov 1996 for a review of the generators of the possible corrugations at that time). New sources of momentum and energy have been incorporated in recent years, which, together with the classical ones, could be summarized as: supernova explosions (Olano 1982), collisions with highvelocity clouds (Franco et al 1988;, encounters with dark matter blobs (Bekki 2009), and interactions with tidal streams (Laporte et al 2018;Ruiz-Lara et al 2020;Tepper-Garcia et al 2022, and references therein), among others. This wide phenomenology can inject, into the interstellar medium, energies higher than 10 51 erg in a single event.…”

“…The hypothesis of an external perturber interacting with the Galaxy, where the main role is attributable to the passage of the Sagittarius dwarf galaxy through the Galactic disk, is gaining more and more strength for explaining the different vertical phase features. One recent example is found in Tepper-Garcia et al (2022), where they study the effects of a single satellite passing through the Galactic disk. They reproduce stellar and gas corrugations, initially in phase, but disentangling after a few rotation periods (500-700 Myr).…”

“…The relative positions of the molecular gas and the YOCs in the solar neighborhood (Figure 13) suggest that this encounter is recent. Although the gas and stars could have been in phase for much longer periods (∼500 Ma), it is more difficult to explain that the maximum deviation from the plane, with amplitude far in excess of the secondary corrugations, was not damped over that time period (Tepper-Garcia et al 2022). The correlation between Z and V Z (Figure 12), the larger amplitudes at Figure 8(c), and the evident spatial connection between the molecular gas and young stars (Figure 13) suggest that the encounter is taking place here and now.…”