Excitation of Coupled Stellar Motions in the Galactic Disk by Orbiting Satellites

Abstract

We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of satellites predicted cosmologically. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, having the appearance of rings in face-on projections of the stellar disk. They also produce flares in the disk outer parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions, with small associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the coherent vertical oscillations recently detected locally. They can also induce non-zero vertical streaming motions as large as 10-20 km s −1 , consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring, with modest associated disk heating.

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

“…The vertical asymmetries in the stellar density may be due to an ancient impact, possibly by the Sagittarius (Sgr) dwarf galaxy (Widrow et al 2012), with support for the impact hypothesis coming not only from theoretical investigations (Purcell et al 2011;Gómez et al 2012a), but also from an observed vertical wave in mean metallicity (An 2019), inferred from SDSS photometry, with features similar to the observed density wave. The novel phase-space structures noted by Antoja et al (2018) also offer support to the impact hypothesis, as such features had been predicted as a consequence (Purcell et al 2011;Gómez et al 2012b;D'Onghia et al 2016;Fux 2001). Recently, too, the discovery of stars with retrograde motion in the disk has led to determination of a previously unidentified ancient impact, from Gaia-Enceladus (or the Gaia-Sausage) in the inner halo Belokurov et al 2018).…”

Implications of a Time-varying Galactic Potential for Determinations of the Dynamical Surface Density

“…Such a wave that propagates through the disk tends to form a two-armed ring-like pattern with over-dense and under-dense regions together with strong bending vertical velocities (see e.g. Gómez et al 2013;D'Onghia et al 2016).…”

“…This feature is interpreted as the incomplete phasemixing phenomenon occurring in the stellar disk and a manifestation of the disk being locally out of dynamical equilibrium. Recent studies using analytic models (Antoja et al 2018;Binney & Schönrich 2018) or numerical simulations (D'Onghia et al 2016;Laporte et al 2018a;Darling & Widrow 2019) interpreted this feature as likely caused by the coupling of vertical and in-plane motions induced by the passage of a satellite galaxy through the Galactic disk. In fact in the presence of rings induced by the impact of a satellite, the intra-ring underdense regions coincide with a local increase of the characteristic scale height (see Fig.…”

“…These motions manisfest as vertical oscillations of the stellar disk as measured by SEGUE, RAVE and LAMOST (e.g., Widrow et al 2012;Williams et al 2013;Xu et al 2015;Carrillo et al 2018;Wang et al 2018b,a) and may be caused by the passage of a massive satellite such as the Sagittarius dwarf galaxy (Gómez et al 2012(Gómez et al , 2013Widrow et al 2014;de la Vega et al 2015;D'Onghia et al 2016), although Faure et al (2014), Debattista (2014), and Monari et al (2016) showed that vertical oscillations in the form of breathing modes are also obtained by the passage of the stars through spiral arms. Several independent studies showed that rings, wobbles and in-plane velocity anisotropies may be the dynamical response of the stellar disk to the gravitational disturbance of a satellite galaxy (Minchev et al 2009;Purcell et al 2011;D'Onghia et al 2016) or their collective effects (Kazantzidis et al 2009;Chequers et al 2018). In the case of a perturbation by the Sagittarius dwarf, not only do such disturbances affect the inner part of the galaxy, but they can impact the outer disk in regions where the stellar surface density decreases and the disk is more sensitive to external perturbations.…”

Effects of coplanar satellite bands on galactic disc evolution

“…Purcell, et al (2011) modeled the effects produced in the Milky Way by the infall of Sgr, yielding spiral wave formation, enhanced bar evolution, and flaring at the edges of the Milky Way's disc. Gómez, et al (2012) and D'Onghia, et al (2016) also simulated the Sgr impact, producing vertical density waves and disc thickening, respectively. Widrow & Bonner (2015) used a similar large satellite to that of Quillen, et al (2009), and in addition to observing the same spirals and thickening as their predecessors, were able to show that the effects of a single perturbance are quickly damped away within the disc and repeated, cyclic perturbances are necessary to induce long-term effects.…”