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

TL;DR

This study investigates the presence and characteristics of breaks in the thin and thick stellar discs of 70 edge-on galaxies using Spitzer imaging, revealing different truncation mechanisms and their relation to galaxy mass and dynamics.

Contribution

It provides the first detailed analysis of separate breaks in thin and thick discs in edge-on galaxies, highlighting different truncation mechanisms and their dependence on galaxy properties.

Findings

Thin discs often truncate (77%), thick discs less so (31%)

When present, break radii are similar in thin and thick discs

Thin disc antitruncations are overestimated due to superposition effects

Abstract

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, 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.