Modelling the spectral energy distribution of galaxies IV.: Correcting apparent disk scalelengths and central surface brightnesses for the effect of dust at optical and near-infrared wavelengths

We present corrections for the change in the apparent scalelengths, central surface brightnesses and axis ratios due to the presence of dust in pure disk galaxies, as a function of inclination, central face-on opacity in the B-band (tau(f)(B)) and wavelength. The correction factors were derived from simulated images of disk galaxies created using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the observed surface-brightness distributions in both the optical/Near-infrared and FIR/submm. We found that dust can significantly affect both the scalelength and central surface brightness, inducing variations in the apparent to intrinsic quantities of up to 50% in scalelength and up to 1.5 mag in central surface brightness. We also identified some astrophysical effects for which, although the absolute effect of dust is non-negligible, the predicted variation over a likely range in opacity is relatively small, such that an exact knowledge of opacity is not needed. Thus, for a galaxy at a typical inclination of 37 degrees and having any tau(f)(B) > 2, the effect of dust is to increase the scalelength in B relative to that in I by a factor of 1.12 +/- 0.02 and to change the B-I central colour by 0.36 +/- 0.05 mag. Finally we use the model to analyse the observed scalelength ratios between B and I for a sample of disk-dominated spiral galaxies, finding that the tendency for apparent scalelength to increase with decreasing wavelength is primarily due to the effects of dust.