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Breaks in the radial luminosity profiles of galaxies have been until nowmostly studied averaged over discs. Here we study separately breaks in thin andthick discs in 70 edge-on galaxies using imaging from the Spitzer Survey ofStellar Structure in Galaxies. We built luminosity profiles of the thin and thethick discs parallel to midplanes and we found that thin discs often truncate(77%). Thick discs truncate less often (31%), but when they do, their breakradius is comparable with that in the thin disc. This suggests either twodifferent truncation mechanisms - one of dynamical origin affecting both discssimultaneously and another one only affecting the thin disc - or a singlemechanism that creates a truncation in one disc or in both depending on somegalaxy property. Thin discs apparently antitruncate in around 40% of galaxies.However, in many cases, these antitruncations are an artifact caused by thesuperposition of a thin disc and a thick disc with the latter having a longerscale length. We estimate the real thin disc antitruncation fraction to be lessthan 15%. We found that the ratio of the thick and thin stellar disc mass isroughly constant (0.2<M_T/M_t<0.7) for circular velocities v_c>120 km/s, butbecomes much larger at smaller velocities. We hypothesize that this is due to acombination of a high efficiency of supernova feedback and a slower dynamicalevolution in lower-mass galaxies causing stellar thin discs to be younger andless massive than in higher-mass galaxies.

Breaks in thin and thick discs of edge-on galaxies imaged in the Spitzer Survey of Stellar Structure in Galaxies (S4G)