Paper detail

The reports of thick discs' deaths are greatly exaggerated: thick discs are NOT artefacts caused by diffuse scattered light

Recent studies have made the community aware of scattered light when examining low-surface-brightness galaxy features such as thick discs. In our past studies of the thick discs of edge-on galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) we modelled the point spread function as a Gaussian. We re-examine our results using a revised point spread function model that accounts for extended wings out to more than 2.5arcmin. We study the $3.6μ{\rm m}$ images of 141 edge-on galaxies from the S$^4$G. We decompose the surface brightness profiles of the galaxies perpendicular to their mid-planes assuming that discs are made of two stellar discs in hydrostatic equilibrium. We decompose the axial surface brightness profiles of galaxies to model the central mass concentration - described by a Sérsic function - and the disc - described by a broken exponential disc. Our improved treatment confirms the ubiquity of thick discs. The main difference between our current fits and those presented before is that now the scattered light from the thin disc dominates the surface brightness at levels below $μ\sim26\,{\rm mag\,arcsec^{-2}}$. This does not affect drastically any of our previously presented results: 1) Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 2) Thick discs have masses comparable to those of thin discs in low-mass galaxies - circular velocities $v_{\rm c}<120\,{\rm km\,s^{-1}}$ - whereas they are typically less massive than the thin discs in high-mass galaxies. 3) Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir. 4) Roughly 60% of the up-bending breaks in face-on galaxies are caused by the superposition of a thin and a thick disc where the scale-length of the latter is the largest. (Abridged)