Do Galactic Bars Buckle to Form Bulges?

The Milky Way is one of many galaxies that has a peanut-shaped bulge at its center. A new study has now caught two galaxies in the process of forming similar bulges, yielding insight into how ours was created.

Unstable Buckling

Roughly 60-70% of disk galaxies in the local universe have stellar bars at the centers of their disks. Many of these — including our own galaxy — are vertically thickened in their inner regions, giving their bulges a boxy or peanut-shaped appearance in an edge-on view. We call these “B/P bulges”.

What causes B/P bulges? Twenty years of simulations of galaxy formation and evolution have pointed to an answer: galactic bars in simulations can buckle, due to a vertical instability that can occur in the bar shortly after its formation. When this asymmetric buckling eventually ends, the inner part of the bar settles into a vertically symmetric structure again: the B/P bulge.

But despite the fact that simulations predict this formation mechanism, we’ve yet to confirm it observationally. Though we’ve observed many examples in the universe of galaxies with boxy bulges that match the outcomes of the simulations, we’ve never yet caught a galactic bar in the act of buckling … until now.

Matching Observation to Simulation

Scientists Peter Erwin (Max Planck Institute for Extraterrestrial Physics, Germany) and Victor Debattista (University of Central Lancashire, UK) searched through barred disk galaxies with the Spitzer Space Telescope, looking for buckling galactic bars. Their search was successful: two galaxies, NGC 4569 and NGC 3227, have the central characteristics of buckling bars!

The authors made this identification by comparing their observations of galaxies to simulated galaxies that were undergoing bar buckling. Several characteristics — like trapezoidal bulge structure and spurs that extend symmetrically off of the long end of the trapezoid — are specifically characteristic of bars that are in the process of buckling. NGC 4569 and NGC 3227 both nicely match these morphological predictions from simulations.

Examining the stellar motions in the center of NGC 4569, Erwin and Debattista additionally find that the stellar kinematics match the specific predictions from simulations of a buckling bar as well.

A Common Structure

Erwin and Debattista’s overall survey results indicate that B/P bulges are extremely common in high-stellar-mass galaxies: they are present in ~80% of the 44 high-stellar-mass barred-disk galaxies they examined. Based on these observations, the fraction of high-mass barred galaxies with bars in the process of buckling is estimated to be ~4.5% in the local universe.

In contrast, the authors calculate that the fraction of galaxies with buckling bars should be much higher in the earlier universe — the buckling fraction peaks at ~40% at a redshift of z = 0.7. The James Webb Space Telescope should be up to the task of detecting these galaxies, so future observations will provide a useful test of the authors’ model for B/P bulge formation.

Citation

Peter Erwin and Victor P. Debattista 2016 ApJ 825 L30. doi:10.3847/2041-8205/825/2/L30