Formation of Warped Disks by Galactic Fly-by Encounters. I. Stellar Disks

TL;DR

This study uses N-body simulations to demonstrate that galactic fly-by encounters can generate long-lasting S-shaped stellar disk warps, explaining various observed warp morphologies and their dependence on encounter parameters.

Contribution

It provides a detailed analysis of how fly-by parameters influence warp formation and suggests multiple interactions can produce complex warp shapes.

Findings

S-shaped warps can be excited and sustained for billions of years.

Warp properties depend on impact parameter, mass ratio, and incident angle.

Multiple fly-bys can produce U- and L-shaped warps through superposition.

Abstract

Warped disks are almost ubiquitous among spiral galaxies. Here we revisit and test the `fly-by scenario' of warp formation, in which impulsive encounters between galaxies are responsible for warped disks. Based on N-body simulations, we investigate the morphological and kinematical evolution of the stellar component of disks when galaxies undergo fly-by interactions with adjacent dark matter halos. We find that the so-called `S'-shaped warps can be excited by fly-bys and sustained for even up to a few billion years, and that this scenario provides a cohesive explanation for several key observations. We show that disk warp properties are governed primarily by the following three parameters; (1) the impact parameter, i.e., the minimum distance between two halos, (2) the mass ratio between two halos, and (3) the incident angle of the fly-by perturber. The warp angle is tied up with all three parameters, yet the warp lifetime is particularly sensitive to the incident angle of the perturber. Interestingly, the modeled S-shaped warps are often non-symmetric depending on the incident angle. We speculate that the puzzling U- and L-shaped warps are geometrically superimposed S-types produced by successive fly-bys with different incident angles, including multiple interactions with a satellite on a highly elongated orbit.