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