Disc distortion revisited

TL;DR

This paper revisits the dynamics of self-gravitating galactic discs, demonstrating rapid evolution of warps into spiral waves, and links observed galactic warps to past interactions like the Sagittarius dwarf galaxy's pericentre.

Contribution

It provides a fast, efficient method to model disc warp dynamics and connects observed galactic warps to historical galactic interactions, improving understanding of disc evolution.

Findings

Warp evolves into a spiral wave that winds up quickly.

Galactic warp caused by Sagittarius dwarf matches observed HI warp.

Model warp amplitude is smaller than observed, but reasons are discussed.

Abstract

We revisit the dynamics of razor-thin, stone-cold, self-gravitating discs. By recasting the equations into standard cylindrical coordinates, the linearised vertical dynamics of an exponential disc can be followed for several gigayears on a laptop in a few minutes. An initially warped disc rapidly evolves into a flat inner region and an outward-propagating spiral corrugation wave that rapidly winds up and would quickly thicken a disc with non-zero radial velocity dispersion. The Sgr dwarf galaxy generates a similar warp in the Galactic disc as it passes through pericentre, and the warp generated by the dwarf's last pericentre ~ 35 Myr ago is remarkably similar to the warp traced by the Galaxy's HI disc. The resemblance to the observed warp is fleeting but its timing is perfect. For the adopted parameters the amplitude of the model warp is a factor 3 too small, but there are several reasons for this being so. The marked flaring of our Galaxy's low-alpha disc just outside the solar circle can be explained as a legacy of earlier pericentres.