Bogus dust screens from well-mixed exponential discs in galaxies

Abstract

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 near side is systematically screened by dust. Such a preferred orientation for dust screens is unlikely. Here we show that common extinction from the embedded dust layer in an exponential disc has the same effect, making the near side systematically redder as the inclination increases. The galaxy NGC 2841 is modelled as an example, where the V - K profile is profoundly asymmetric and actually step-like across the centre. We predict that the minor-axis emission profile of the same dust in the far-infrared, at wavelength λ ∼ 200 μm, will be much more symmetric than the optical profiles, implying nearly equal column densities of dust on both sides of the minor axis.