A Universal Dance of Galactic Disks: Ubiquitous Precession and Its Implications

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Abstract
Precession is a very common phenomenon for small-scale astronomical objects. However, the precession of galactic disks, occurring on scales of kiloparsecs or more, has been insufficiently studied in past works. Quantifying this precession observationally remains challenging due to the lack of high-resolution dynamical data. Cosmological simulations, in which gravitational interactions are self-consistently modeled, offer a unique avenue for investigating disk precession. Leveraging the IllustrisTNG simulations, we trace the evolution of the spin orientation in Milky Way–like galaxies over cosmic time. We find that disk precession is ubiquitous in galaxies and significantly affects galaxy evolution. It is driven by the external tidal torque originating from the anisotropic matter distribution within 30 kpc, and is violent at z > 1, becoming gentler but still significant by z  ∼ 0, when disks are considered dynamically settled. Disk precession can induce significant warping of cold gas, which is often observed in the Milky Way and nearby galaxies. We predict that the Milky Way is currently precessing at a rate of ≃3°–10° Gyr –1 based on its observed warp. Violent precession can heat the orbits of stars, which may eventually produce prolate elliptical galaxies. The tidal torque from central galaxies can cause the precession of nearby satellite galaxies, causing their disk planes to point toward the centrals, which explains the observed radial alignment. We also find that the precession of accreted cold gas streams, regulated by galactic torques, is crucial for the evolution of disk galaxies.

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