Breaks in the radial luminosity profiles of galaxies have been until now mostly studied averaged over discs .
Here we study separately breaks in thin and thick discs in 70 edge-on galaxies using imaging from the Spitzer Survey of Stellar Structure in Galaxies .
We built luminosity profiles of the thin and the thick discs parallel to midplanes and we found that thin discs often truncate ( 77 % ) .
Thick discs truncate less often ( 31 % ) , but when they do , their break radius is comparable with that in the thin disc .
This suggests either two different truncation mechanisms – one of dynamical origin affecting both discs simultaneously and another one only affecting the thin disc – or a single mechanism that creates a truncation in one disc or in both depending on some galaxy property .
Thin discs apparently antitruncate in around 40 % of galaxies .
However , in many cases , these antitruncations are an artifact caused by the superposition of a thin disc and a thick disc with the latter having a longer scale length .
We estimate the real thin disc antitruncation fraction to be less than 15 % .
We found that the ratio of the thick and thin stellar disc mass is roughly constant ( 0.2 < M _ { T } / M _ { t } < 0.7 ) for circular velocities v _ { c } > 120 { km s ^ { -1 } } , but becomes much larger at smaller velocities .
We hypothesize that this is due to a combination of a high efficiency of supernova feedback and a slower dynamical evolution in lower-mass galaxies causing stellar thin discs to be younger and less massive than in higher-mass galaxies .