We produce three-dimensional Monte-Carlo radiative transfer models of the edge-on spiral galaxy NGC 891 , a fast-rotating galaxy thought to be an analogue to the Milky Way .
The models contain realistic spiral arms and a fractal distribution of clumpy dust .
We fit our models to Hubble Space Telescope images corresponding to the B and I bands , using shapelet analysis and a genetic algorithm to generate 30 statistically best-fitting models .
These models have a strong preference for spirality and clumpiness , with average face-on attenuation decreasing from 0.24 ( 0.16 ) to 0.03 ( 0.03 ) mag in the B ( I ) band between 0.5 and 2 radial scale-lengths .
Most of the attenuation comes from small high-density clumps with low ( \lesssim 10 % ) filling factors .
The fraction of dust in clumps is broadly consistent with results from fitting NGC 891 ’ s spectral energy distribution .
Because of scattering effects and the intermixed nature of the dust and starlight , attenuation is smaller and less wavelength-dependent than the integrated dust column-density .
Our clumpy models typically have higher attenuation at low inclinations than previous radiative transfer models using smooth distributions of stars and dust , but similar attenuation at inclinations above 70 ^ { \circ } .
At all inclinations most clumpy models have less attenuation than expected from previous estimates based on minimizing scatter in the Tully-Fisher relation .
Mass-to-light ratios are higher and the intrinsic scatter in the Tully-Fisher relation is larger than previously expected for galaxies similar to NGC 891 .
The attenuation curve changes as a function of inclination , with R _ { B,B - I } = \frac { A _ { B } } { E ( B - I ) } increasing by \sim 0.75 from face-on to near-edge-on orientations .