  We have analyzed the radial distribution of old stars in a sample of 218
nearby face-on disks, using deep 3.6um images from the Spitzer Survey of
Stellar Structure in Galaxies (S4G). In particular, we have studied the
structural properties of those disks with a broken or down-bending profile. We
find that, on average, disks with a genuine single exponential profile have a
scale-length and a central surface brightness which are intermediate to those
of the inner and outer components of a down-bending disk with the same total
stellar mass. In the case of barred galaxies, the ratio between the break and
the bar radii (Rbr/Rbar) depends strongly on the total stellar mass of the
galaxy. For galaxies more massive than 10^10 Msun, the distribution is bimodal,
peaking at Rbr/Rbar~2 and ~3.5. The first peak, which is the most populated
one, is linked to the Outer Lindblad Resonance of the bar, whereas the second
one is consistent with a dynamical coupling between the bar and the spiral
pattern. For galaxies below 10^10 Msun, breaks are found up to ~10 Rbar, but we
show that they could still be caused by resonances given the rising nature of
rotation curves in these low-mass disks. While not ruling out star formation
thresholds, our results imply that radial stellar migration induced by
non-axysymmetric features can be responsible not only for those breaks at 2
Rbar, but also for many of those found at larger radii.
