  We study the relative dust attenuation-inclination relation in 78,721 nearby
galaxies using the axis ratio dependence of optical-NIR colour, as measured by
the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and
the Wide-field Infrared Survey Explorer (WISE). In order to avoid to the
greatest extent possible attenuation-driven biases, we carefully select
galaxies using dust attenuation-independent near- and mid-IR luminosities and
colours. Relative u-band attenuation between face-on and edge-on disc galaxies
along the star forming main sequence varies from ~0.55 mag up to ~1.55 mag. The
strength of the relative attenuation varies strongly with both specific star
formation rate and galaxy luminosity (or stellar mass). The dependence of
relative attenuation on luminosity is not monotonic, but rather peaks at
$M_{3.4\mu m} \approx -21.5$, corresponding to $M_* \approx 3\times
10^{10}M_{Sun}$. This behavior stands seemingly in contrast to some older
studies; we show that older works failed to reliably probe to higher
luminosities, and were insensitive to the decrease in attenuation with
increasing luminosity for the brightest star-forming discs.
Back-of-the-envelope scaling relations predict the strong variation of dust
optical depth with specific star formation rate and stellar mass. More in-depth
comparisons using the scaling relations to model the relative attenuation
require the inclusion of star-dust geometry to reproduce the details of these
variations (especially at high luminosities), highlighting the importance of
these geometrical effects.
