Caught in the Act: Direct Detection of Galactic Bars in the Buckling Phase

TL;DR

This paper provides the first direct observational evidence of the buckling phase in galactic bars, confirming a key process in the formation of boxy/peanut-shaped bulges in real galaxies.

Contribution

It is the first to identify ongoing buckling in real galaxies, validating long-standing simulation predictions about bar evolution.

Findings

Detected asymmetric isophotes indicating buckling

Observed stellar-kinematic asymmetries consistent with simulations

Estimated buckling phase duration of 0.5--1 Gyr

Abstract

The majority of massive disk galaxies, including our own, have stellar bars with vertically thick inner regions -- so-called "boxy/peanut-shaped" (B/P) bulges. The most commonly suggested mechanism for the formation of B/P bulges is a violent vertical "buckling" instability in the bar, something that has been seen in N-body simulations for over twenty years, but never identified in real galaxies. Here, we present the first direct observational evidence for ongoing buckling in two nearby galaxies (NGC 3227 and NGC 4569), including characteristic asymmetric isophotes and (in NGC 4569) stellar-kinematic asymmetries that match buckling in simulations. This confirms that the buckling instability takes place and produces B/P bulges in real galaxies. A toy model of bar evolution yields a local fraction of buckling bars consistent with observations if the buckling phase lasts ~0.5--1 Gyr, in agreement with simulations.