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5.4. Diffuse Dust

Zwicky [168] noted a deficiency of clusters behind the Coma cluster as compared to other regions of the sky. He interpreted this deficiency as evidence for IC extinction, presumably due to diffuse dust. Later, Noonan [101] found evidence for IC extinction of ~ 0.4 mag in the blue band, in agreement with the estimate given by Karachentsev & Lipovetskii [77]. Few years later, Wesson [151] made the rather extreme hypothesis that IC dust may be present in such large quantities as to bind the Coma cluster!

The IC dust hypothesis encountered more criticism than consensus. In the 60's de Vaucouleurs noted that the fact that diffuse gas remained undetected implied a low density of diffuse dust, and Abell [4] considered the evidence for IC inconclusive. Smart [125] showed that IC dust must be significantly depleted because of the dust grain sputtering by the hot IC gas. Tifft & Gregory [138] identified a group in the background of Coma and showed that the magnitudes of galaxies in this group are not significantly affected by extinction.

Recently, Dwek et al. [41] modelled the formation and evolution of IC dust in Coma, including sputtering from the IC gas and dust injection from galaxies. They derived a dust density much below that required to explain the observed visual extinction, but consistent with the upper limit reported by IRAS for IR emission in the Coma region. Ferguson [46], using the Mg2-(B-V) relation for Coma ellipticals, set an upper limit of E(B-V) ~ 0.05 for IC extinction.

The amount of IC dust recently detected with ISO (Stickel, these proceedings) is even lower than what predicted by Dwek et al.'s model.

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