A unified picture of breaks and truncations in spiral galaxies from SDSS and S^4G imaging

The mechanism causing breaks in the radial surface-brightness distribution of spiral galaxies is not yet well known. Despite theoretical efforts, there is not a unique explanation for these features and the observational results are not conclusive. In an attempt to address this problem, we have selected a sample of 34 highly inclined spiral galaxies present in both the Sloan Digital Sky Survey (SDSS) and the Spitzer Survey of Stellar Structure in Galaxies (S^4G). We have measured the surface-brightness profiles in the five Sloan optical bands and in the 3.6-μm Spitzer band. We have also calculated the colour and stellar surface mass density profiles using the available photometric information, finding two differentiated features: an innermost break radius at distances of ∼8 ± 1 kpc [0.77 ± 0.06 R_(25)] and a second characteristic radius, or truncation radius, close to the outermost optical extent (∼14 ± 2 kpc [1.09 ± 0.05 R_(25)]) of the galaxy. We propose in this work that breaks might be phenomena related to a threshold in the star formation, while truncations are more likely a real drop in the stellar mass density of the disc associated with the maximum angular momentum of the stars