Why are some galaxies not barred?
  Although more than two-thirds of star-forming disk galaxies in the local
universe are barred, some galaxies remain un-barred, occupying the upper half
of the Hubble tuning fork diagram. Numerical simulations almost always produce
bars spontaneously, so it remains a challenge to understand how galaxies
sometimes prevent bars from forming. Using a set of collisionless simulations,
we first reproduce the common result that cold stellar disks surrounding a
classical bulge become strongly unstable to non-axisymmetric perturbations,
leading to the rapid formation of spiral structure and bars. However, our
analyses show that galaxy models with compact classical bulges (whose average
density is greater than or comparable to the disk density calculated within
bulge half-mass radii) are able to prevent bar formation for at least 4 Gyr
even when the stellar disk is maximal and having low Toomre Q. Such bar
prevention is the result of several factors such as (a) a small inner Lindblad
resonance with a high angular rate, which contaminates an incipient bar with
$x_2$ orbits, (b) rapid loss of angular momentum accompanied by a rapid heating
in the center from initially strong bar and spiral instabilities in a low-Q
disk, in other words, a rapid initial rise to a value larger than $\sim5$ of
the ratio of the random energy to the rotational energy in the central region
of the galaxy.
