Bogus dust screens from well-mixed exponential discs in galaxies

The V - K colours along the minor axes of spiral galaxies typically change from red to blue with increasing distance, giving the impression that the n ear side is systematically screened by dust, Such a preferred orientation f or dust screens is unlikely, Here we show that common extinction from the e mbedded dust layer in an exponential disc has the same effect, making the n ear side systematically redder as the inclination increases. The galaxy NGC 2841 is modelled as an example, where the V - K profile is profoundly asym metric and actually step-like across the centre, We predict that the minor- a;sis emission profile of the same dust in the far-infrared, at wavelength lambda - 200 mu m, will be much more symmetric than the optical profiles, i mplying nearly equal column densities of dust on both sides of the minor ax is.