A unified picture of breaks and truncations in spiral galaxies from SDSS and S4G 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 (S4G). 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.