2025MNRAS.542..688K

We analyse the stellar light, 4000 Å break and emission line profiles of 82 edge-on disc galaxies from the Mapping Nearby Galaxies at APO (MaNGA) survey. We characterize the stellar light profiles perpendicular to the disc plane using two parameters: (a) the power-law slope of the thick disc component and (b) the transition radius where the profile flattens. The 4000 Å break profiles perpendicular to the plane are characterized by the number of significant changes in slope (breaks) and by the change in D$_n$(4000) from inner to outer disc. The slope correlates tightly with the stellar mass of the galaxy over the stellar mass range $10^9< \log M_* < 10^{10} \,\mathrm{ M}_{\odot }$. More massive galaxies have more extended thick discs. The slope and transition radius exhibit large scatter for galaxies more massive than $10^{10} \,\mathrm{ M}_{\odot }$. Half the sample have older stellar populations in their thick discs, a third have flat D$_n$(4000) profiles and 15 per cent have younger thick discs. The D$_n$(4000) profiles exhibit as many as four separate breaks. There are more breaks in massive galaxies with bulges and more extended thick discs. This may indicate that the breaks are produced by more frequent accretion events in such systems. The extraplanar H$\alpha$ equivalent width (EQW) correlates most strongly with the specific star formation rate (SFR) of the galaxy, and the [O II]/H$\alpha$ ratio increases with distance from the disc plane. This increase is most apparent for massive galaxies with extended thick disc components and low SFR/$M_*$. These findings support the hypothesis that the larger [O II]/H$\alpha$ ratios may be caused by ionization from evolved stars.