A Test for Dust in Clusters of Galaxies

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

A number of studies over the last 35 years have suggested that the counts of distant galaxies and quasars may be modulated by the presence of dust in intervening galaxy clusters. Required extinctions are in the range 0.2 < A_B_ < 0.5, widely distributed within the clusters. Extinction should also affect the colours of galaxies, and this reddening is potentially detectable in existing data, if the amount of extinction spread throughout nearby clusters is as large as that proposed (in more distant clusters) to explain the background counts. To test for reddening, we examine the relation between the Mg_2_ index, which is insensitive to reddening, and B - V colours, which are affected by reddening, for a large sample of elliptical galaxies. We find no difference In the Mg_2_ versus B - V correlation for cluster, group and field samples, and can constrain the mean reddening to be less than E(B - V) = 0.06 mag at the 90 percent confidence level In a composite sample of 19 nearby clusters. The upper limits are E(B - V) = 0.06 and 0.05 mag respectively, for the Virgo and Coma clusters. Dust may also be expected to increase the scatter in Mg_2_ versus B - V, as a result of both patchy obscuration, and the fact that the galaxies are viewed through varying depths in the clusters. We find that the distribution of the residuals about the mean relation is the same for the cluster, group and field samples. Our results suggest that dust is not present in clusters of galaxies in the quantities needed to explain the anticorrelations in the quasar and galaxy counts.

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