Bending waves in the Milky Way’s disc from halo substructure

Abstract

We use N-body simulations to investigate the excitation of bending waves in a Milky Way-like disc-bulge-halo system. The dark matter halo consists of a smooth component and a population of subhaloes while the disc is composed of thin and thick components. Also considered is a control simulation where all of the halo mass is smoothly distributed. We find that bending waves are more vigorously excited in the thin disc than the thick one and that they are strongest in the outer regions of the disc, especially at late times. By way of a Fourier decomposition, we find that the complicated pattern of bending across the disc can be described as a superposition of waves, which concentrate along two branches in the radius-rotational frequency plane. These branches correspond to vertical resonance curves as predicted by a WKB analysis. Bending waves in the simulation with substructure have a higher amplitude than those in the smooth-halo simulation, though the frequency-radius characteristics of the waves in the two simulations are very similar. A cross correlation analysis of vertical displacement and bulk vertical velocity suggests that the waves oscillate largely as simple plane waves. We suggest that the wave-like features in astrometric surveys such as the Second Data Release from Gaia may be due to long-lived waves of a dynamically active disc rather than, or in addition to, perturbations from a recent satellite-disc encounter.

The GALAH survey and Gaia DR2: Linking ridges, arches, and vertical waves in the kinematics of the Milky Way

“…Nevertheless, the total stellar disc mass is consistent with other estimates (e.g., Bovy & Rix 2013). Overall, our choices of component profiles and the values of their corresponding parameters define a valid model for the Galaxy, comparable to the model successfully adopted by others in numerical studies (e.g., Chequers et al 2018). Our interaction models consist essentially of a twocomponent (DM, stellar spheroid) system orbiting an initially isolated Galaxy along an (unrealistic) hyperbolic orbit.…”

Footprints of the Sagittarius dwarf galaxy in the Gaia data set

“…Springel et al 2008). Their collective effect of the subhalo mass function has been proposed to excite bending waves in the disc in numerical experiments by Chequers et al (2018), however we note that these used initial conditions which did not model this effect correctly. Indeed such experiments have used velocity distribution functions similar to that of the underlying parent halo, which is not representative of cosmologically formed Galactic dark matter halos (Sawala et al 2017) Figure 17.…”

Applying Noether’s Theorem to Matter in the Milky Way: Evidence for External Perturbations and Non-steady-state Effects from Gaia Data Release 2

“…Studies of flaring HI gas in the outer galaxy also support a prolate DM distribution (Banerjee & Jog 2011); these authors note that a prolate halo can support long-lived warps (Ideta et al 2000), which would help to explain why they are commonly seen (Banerjee & Jog 2011). It has also been suggested that some of these features could arise from a dynamically active disk (Chequers et al 2018) in isolation. Others note that ridges in phase-space may also connect to the Galactic bar (Mühlbauer & Dehnen 2003;Fragkoudi et al 2019).…”