Cosmological obscuration by galactic dust: effects of dust evolution

We explore the effects of dust in cosmologically distributed intervening ga laxies in a high-redshift universe using a generalized model where dust con tent evolves with cosmic time. The absorbing galaxies are modelled as expon ential discs which form coevally, maintain a constant space density and evo lve in dust content at a rate that is uniform throughout. We find that the inclusion of moderate to moderately weak amounts of evolution consistent wi th other studies can reduce the mean observed B-band optical depth to redsh ifts z greater than or similar to 1 by at least 60 per cent relative to non -evolving models. Our predictions imply that intervening galactic dust is u nlikely to bias the optical counts of quasars at high redshifts and their d erived evolution in space density.