On the varied origins of up-bending breaks in galaxy disks
  Aims: Using a sample of 175 low-inclination galaxies from the S$^{4}$G, we
investigate the origins of up-bending (Type III) breaks in the 3.6 $\mu$m
surface brightness profiles of disk galaxies.
  Methods: We re-analyze a sample of previously identified Type III disk
break-hosting galaxies using a new, unbiased break-finding algorithm, which
uncovered many new, sometimes subtle disk breaks across the whole sample. We
classify each break by its likely origin through close examination of the
galaxy images across wavelengths, and compare samples of galaxies separated by
their outermost identified break types in terms of their stellar populations
and local environments.
  Results: We find that more than half of the confirmed Type III breaks in our
sample can be attributed to morphological asymmetry in the host galaxies. As
these breaks are mostly an artifact of the azimuthal averaging process, their
status as physical "breaks" is questionable. Such galaxies occupy some of the
highest density environments in our sample, implying that much of this
asymmetry is the result of tidal disturbance. Additionally, we find that Type
III breaks related to extended spiral arms or star formation often host
down-bending (Type II) breaks at larger radius which were previously
unidentified. Such galaxies reside in the lowest density environments in our
sample, in line with previous studies that found a lack of Type II breaks in
clusters. Galaxies occupying the highest density environments most often show
Type III breaks associated with outer spheroidal components.
  Conclusions: We find that Type III breaks in the outer disks of galaxies
arise most often through environmental influence: either tidal disturbance
(resulting in disk asymmetry) or heating through, e.g., galaxy harrassment
(leading to spheroidal components). Galaxies hosting the latter break types
also show... (abstract continues)
