THE VARIATION OF THE SCALELENGTHS OF GALAXIES AT DIFFERENT WAVELENGTHS

We develop model profiles of spiral galaxies with different amounts and geometries of disc absorption and different bulge sizes. These are used to investigate whether there are any systematic changes in the profiles of absorbed galaxies at different wavelengths, and whether measurement of the scalelength of disc galaxies at different wavelengths can be used as a test of whether galaxies are optically thick or thin. Using models with realistic geometries where the dust vertical scaleheight is less than the stars' vertical scaleheight, and including a bulge component, we find very little difference in the B-band and I- or II-band scalelengths, even for optical depths as high as tau(B) = 20. When we compare the predictions of our models to our own data and the data in the literature, we find that the observational uncertainties are greater than the predicted effects, rendering this test of optical depth essentially useless. However, our models do predict subtle trends and differences in the surface brightness profiles of galaxies when viewed at different wavelengths. With the increasing availability of near-infrared arrays, it should be possible to test whether these predicted differences are, in fact, seen.