Rings and spiral arms: are they coupled with bars?
  Rings and spiral arms are distinctive features of many galaxies, and their
properties are closely related to the disk dynamics. They are often associated
to stellar bars, but the details of this connection are far from clear. We
study the pitch angles of spiral arms and the frequency and dimensions of inner
and outer rings as a function of disk parameters and the amplitude of
non-axisymmetries in the S$^4$G survey. The ring fraction increases with bar
Fourier density amplitude: this can be interpreted as evidence for the role of
bars in ring formation. The sizes of inner rings, normalised by the disk size,
are positively correlated with bar strength: this can be linked to the radial
displacement of the inner 4:1 ultra-harmonic resonance while the bar grows and
the pattern speed decreases. The fraction of rings is larger in barred galaxies
than in their non-barred counterparts, but still $\sim 1/3$ ($\sim 1/4$) of the
galaxies hosting inner (outer) rings are not barred. The amplitudes of bars and
spirals are correlated for all types of spirals. However, on average, the pitch
angles of spiral arms are roughly the same for barred and non-barred galaxies:
this questions the role of bars exciting spiral structure. We conclude that the
present-day coupling of rings, spiral arms, and bars is not as robust as
predicted by simulations.
