Bar instability in disk-halo systems
  We show that the exponential growth rate of a bar in a stellar disk is
substantially greater when the disk is embedded in a live halo than in a rigid
one having the same mass distribution. We also find that the vigor of the
instability in disk-halo systems varies with the shape of the halo velocity
ellipsoid. Disks in rigid halos that are massive enough to be stable by the
usual criteria, quickly form bars in isotropic halos and much greater halo mass
is needed to avoid a strong bar; thus stability criteria derived for disks in
rigid halos do not apply when the halo is responsive. The study presented here
is of an idealized family of models with near uniform central rotation and that
lack an extended halo; we present more realistic models with extended halos in
a companion paper. The puzzle presented by the absence of strong bars in some
galaxies having gently rising inner rotation curves is compounded by the
results presented here.
