Building the peanut: simulations and observations of peanut-shaped structures and ansae in face-on disk galaxies

TL;DR

This paper presents simulations and observations of peanut-shaped structures in face-on disk galaxies, revealing their formation, morphology, and kinematic features, and compares these with real galaxy data.

Contribution

It demonstrates that peanut-shaped structures can be seen in face-on views through simulations, and links their properties to observable features like ansae and rings, expanding understanding of galaxy morphology.

Findings

Peanut structures are visible in face-on projections in simulations.

Peanut length correlates with bar length, peaking at half the bar.

Kinematic features such as cylindrical rotation and velocity pinches are identified.

Abstract

(X/peanut)-shaped features observed in a significant fraction of disk galaxies are thought to have formed from vertically buckled bars. Despite being three dimensional structures, they are preferentially detected in near edge-on projection. Only a few galaxies are found to have displayed such structures when their disks are relatively face-on - suggesting that either they are generally weak in face-on projection or many may be hidden by the light of their galaxy's face-on disk. Here we report on three (collisionless) simulated galaxies displaying peanut-shaped structures when their disks are seen both face-on and edge-on - resembling a three-dimensional peanut or dumbbell. Furthermore, these structures are accompanied by ansae and an outer ring at the end of the bar --- as seen in real galaxies such as IC~5240. The same set of quantitative parameters used to measure peanut structures in real galaxies have been determined for the simulated galaxies, and a broad agreement is found. In addition, the peanut length grows in tandem with the bar, and is a maximum at half the length of the bar. Beyond the cutoff of these peanut structures, towards the end of the bar, we discover a new positive/negative feature in the $B_6$ radial profile associated with the isophotes of the ansae/ring. Our simulated, self-gravitating, three-dimensional peanut structures display cylindrical rotation even in the near-face-on disk projection. In addition, we report on a kinematic pinch in the velocity map along the bar minor-axis, matching that seen in the surface density map.