Data analysis
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First, we aligned the images in all of the $B, V, I$, and $K_{\rm s}$ filters. We took special care to subtract foreground stars. This is necessary and important because foreground stars projected close to a galaxy can contaminate the galaxy’s luminosity distribution and lead to erroneous results. We used a master mask constructed by adding the individual masks for each of the passbands. In each mask, the stars were marked with circles of radius around twice the FWHM to avoid contributions from any residual starlight. In some galaxies observed near the Galactic plane, this meant that a large number of pixels had to be masked.

To determine the warp curve (tracing the position of the intensity maxima as a function of radius), we employed three different methods.

1.  First, we calculated the intensity maxima using vertical cuts (perpendicular to the galactic plane). This resulted in very noisy profiles for the optical data and also sometimes for the NIR data, because the presence of a dust lane frequently resulted in a double-peaked luminosity distribution.

2.  Second, we calculated the luminosity distribution’s first moment, with which we obtained reliable results (although it also depended on the position of 