a1 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
a2 Helsinki University Observatory, PO Box 14, 00014 Helsinki, Finland
Strip scanning measurements with crossing position angles have been carried out at 120 μm and 180 μm with the ISOPHOT instrument aboard ISO to observe extended FIR emission from six Abell clusters. The raw I120 μm/I180 μm surface brightness ratio including zodiacal light shows a bump towards Abell 1656 (Coma), dips towards Abell 262 and Abell 2670, and is without an unambiguous feature towards Abell 400, Abell 496, and Abell 4038. The subtraction of the zodiacal light leaves the bump towards Abell 1656 still present, while the dips towards Abell 262 and Abell 2670 are no longer discernible. This behavior can be reconciled in Abell 1656 with a localized excess of emitting material outside the Galaxy with properties different from the galactic foreground cirrus, and in Abell 262 and Abell 2670 with galactic cirrus structures localized on the line-of-sight to these clusters. At 120 μm the excess towards Abell 1656 (Coma) is ≈ 0.2 MJy/sr, with an integrated excess flux within the central region of 10'-15' diameter of ≈ 2.8 Jy. It is interpreted as being due to thermal emission from intracluster dust distributed in the hot X-ray emitting intracluster medium. Only a rough estimate of the associated dust mass of MD ≈ 107 M⊙ can be derived, since the dust temperature is poorly constrained. The visual extinction associated with this dust mass is negligible (AV << 0.1 mag) and much smaller than claimed from optical observations. No evidence for intracluster dust is found in the other five clusters observed. The rather low inferred dust mass in Abell 1656 together with the absence of a signature for intracluster dust in the other clusters observed indicates that intracluster dust is likely not responsible for the excess X-ray absorption seen in cooling flow clusters. These observations thereby represent a further unsuccessful attempt in detecting the presumed final stage of the cooling flow material, in accord with quite a number of previous studies. Finally, the observed dimming of the high - redshift supernovae is unlikely be attributable to extinction caused by dust in the intracluster or even a presumed intercluster medium.
(Online publication September 25 2002)