We have analyzed the radial distribution of old stars in a sample of 218 nearby face-on disks, using deep 3.6 µm images from the Spitzer Survey of Stellar Structure in Galaxies (S 4 G). 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 downbending disk with the same total stellar mass. In the particular case of barred galaxies, the ratio between the break and the bar radii (R br /R bar ) depends strongly on the total stellar mass of the galaxy. For galaxies more massive than 10 10 M ⊙ , the distribution is bimodal, peaking at R br /R bar ∼ 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 M ⊙ , breaks are found up to ∼ 10R bar , 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 ∼ 2R bar , but also for many of those found at larger radii.

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The inclination of M31 is too close to edge-on for a bar component to be easily recognised and is not sufficiently edge-on for a boxy/peanut bulge to protrude clearly out of the equatorial plane. Nevertheless, a sufficient number of clues allow us to argue that this galaxy is barred. We use fully self-consistent N-body simulations of barred galaxies and compare them with both photometric and kinematic observational data for M31. In particular, we rely on the near infrared photometry presented in a companion paper. We compare isodensity contours to isophotal contours and the light profile along cuts parallel to the galaxy major axis and offset towards the North, or the South, to mass profiles along similar cuts on the model. All these comparisons, as well as position velocity diagrams for the gaseous component, give us strong arguments that M31 is barred. We compare four fiducial N-body models to the data and thus set constraints on the parameters of the M31 bar, as its strength, length and orientation. Our `best' models, although not meant to be exact models of M31, reproduce in a very satisfactory way the main relevant observations. We present arguments that M31 has both a classical and a boxy/peanut bulge. Its pseudo-ring-like structure at roughly 50' is near the outer Lindblad resonance of the bar and could thus be an outer ring, as often observed in barred galaxies. The shape of the isophotes also argues that the vertically thin part of the M31 bar extends considerably further out than its boxy bulge, i.e. that the boxy bulge is only part of the bar, thus confirming predictions from orbital structure studies and from previous N-body simulations.Comment: 14 pages, 12 figures, minor corrections, accepted by MNRAS. Version with high resolution figures at http://www.oamp.fr/dynamique/pap/M31_th.pd