Conclusions

In this paper, we have shown that dust is more effective at obscuring background sources when diffuse or extended. We find that obscuration of background sources by a given dust distribution with optical depth will be most effective when , where is the cumulative luminosity function slope of the sources.

We have explored the effects of diffuse dust from, first, galaxy clusters and second, from a hypothesised uniform IGM component. By assuming different radial dust density profiles in a typical rich cluster, we have predicted the optical depth and background source deficit as a function of projected cluster radius. These predictions can be compared with future observations to constrain the properties of intracluster dust. Our predicted optical depth measures () satisfy the above criterion () for background luminous QSOs and galaxies. Existing studies claiming anticorrelations in the distribution of QSOs with foreground clusters down to scales are consistent with a dust density profile that follows the galaxy distribution.

As a further illustration, we have explored the effects of a diffuse IGM dust component with cosmic mass density equal to that observed in local galaxies. Assuming this density is constant on comoving scales to z=2, we find a deficit in background sources about three times greater than that predicted assuming dust in normal galaxies alone.

The `diffuseness' of the dust is the key parameter which we claim determines the effectiveness of obscuration of the background universe. Although such dust distributions may be difficult to detect, we must not neglect their possible presence. Further studies of spatial dust distributions, preferably via the counts and colours of background sources will be essential in confirming our predictions.

© Copyright Astronomical Society of Australia 1997