Oscillating dynamical friction on galactic bars by trapped dark matter

Abstract

The dynamic evolution of galactic bars in standard ΛCDM models is dominated by angular momentum loss to the dark matter haloes via dynamical friction. Traditional approximations to dynamical friction are formulated using the linearized collisionless Boltzmann equation and have been shown to be valid in the fast limit, i.e. for rapidly slowing bars. However, the linear assumption breaks down within a few dynamical periods for typical slowly evolving bars, which trap a significant amount of disc stars and dark matter in resonances. Recent observations of the Galactic bar imply this slow regime at the main bar resonances. We formulate the time-dependent dynamical friction in the slow limit and explore its mechanism in the general slow regime with test-particle simulations. Here, angular momentum exchange is dominated by resonantly trapped orbits which slowly librate around the resonances. In typical equilibrium haloes, the initial phase-space density within the trapped zone is higher at lower angular momentum. Since the libration frequency falls towards the separatrix, this density contrast winds up into a phase-space spiral, resulting in a dynamical friction that oscillates with ∼Gyr periods and damps over secular timescales. We quantify the long-term behaviour of this torque with secular perturbation theory, and predict two observable consequences: i) The phase-space spirals may be detectable in the stellar disc where the number of windings encodes the age of the bar. ii) The torque causes weak oscillations in the bar’s pattern speed, overlaying the overall slowdown – while not discussed, this feature is visible in previous simulations.

A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. I. Phase Spirals in an Infinite, Isothermal Slab

“…A net response requires gradients in the (unperturbed) DF with respect to the actions and/or velocities. Similar solutions for the response of perturbed, collisionless systems have been derived in a number of previous studies (e.g., Lynden-Bell & Kalnajs 1972;Tremaine & Weinberg 1984;Carlberg & Sellwood 1985;Weinberg 1989Weinberg , 1991Weinberg , 2004Kaur & Sridhar 2018;Banik & van den Bosch 2021a;Chiba & Schonrich 2022;Kaur & Stone 2022), often in the context of phenomena like angular momentum transport, radial migration, or dynamical friction.…”

Detection of Multiple Phase Space Overdensities of GSE Stars by Orbit Integration

“…However, considering the small pericenter (r peri < 5 kpc) of our K giants and the massive long bulge/bar structure ∼10 10 M e (Portail et al 2015(Portail et al , 2017, the integrated orbits could be heavily perturbed by the bar when passing the Galactic center or resonance trapped regions related to the bar. Previous studies show that the dynamical influence of a rotating bar is not only constrained to the disk stars (Chen et al 2022;Chiba & Schönrich 2022;Li et al 2023) but also reaches to a much further region (Chemel et al 2018). The barlike perturbations is possibly related to the creation and growth of several substructures, such as Hyades, Sirius, Hercules, and Ophiuchus streams (Dehnen 1998;Antoja et al 2014;Hattori et al 2016Hattori et al , 2019.…”

“…We add a steadily rotating bar to the modified MWPoten-tial2014 with M 200 = 10 12 M e by a python package Agama (Vasiliev 2019). To represent the bar, we choose a model following Chiba & Schönrich (2022) as…”

“…where A is the strength of the bar, v c is the circular velocity in the solar vicinity, and b is the ratio between the bar scale length and the value of corotation radius r CR . Following Chiba & Schönrich (2022), we set v c = 235 km s −1 , A = 0.02, b = 0.28, and r CR = 6.7 kpc. The initial phase angle of the bar f b is 28° (Wegg et al 2015).…”

A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. II. Phase Spirals in an Inhomogeneous Disk Galaxy with a Nonresponsive Dark Matter Halo

“…The adiabatic invariance of actions is partially broken near these resonances, causing the stars to get trapped in librating near-resonant orbits. A proper treatment of the near-resonant response can be performed by working with "slow" and "fast" action-angle variables(Tremaine & Weinberg 1984;Lichtenberg & Lieberman 1992;Chiba & Schönrich 2022;Hamilton et al 2022), which are uniquely defined for each resonance as linear combinations of the original action-angle variables. The fast actions remain nearly invariant while the fast angles oscillate with periods comparable to the unperturbed orbital periods of stars.…”