Far-infrared emission from intracluster dust in Abell clusters

The ISOPHOT instrument aboard ISO has been used to observe extended FIR emission of six Abell clusters. Strip scanning measurements with crossing position angles centered on the clusters were carried out at 120 $\mathrm{\mu m}$ and 180 $\mathrm{\mu m}$. The raw profiles of the $I_{\rm 120 ~\mu m} / I_{\rm 180~\mu m}$ surface brightness ratio including zodiacal light show a bump towards Abell 1656 (Coma), dips towards Abell 262 and Abell 2670, and are without clear structure towards Abell 400, Abell 496, and Abell 4038. After subtraction of the zodiacal light, the bump towards Abell 1656 is still present, while the dips towards Abell 262 and Abell 2670 are no longer noticable. This indicates a localized excess of emitting material outside the Galaxy towards Abell 1656 with properties different from the galactic foreground cirrus, while the behavior in Abell 262 and Abell 2670 can be reconciled with galactic cirrus structures localized on the line-of-sight to these clusters. The excess of ≈ 0.2 MJy/sr seen at 120 $\mathrm{\mu m}$ towards Abell 1656 (Coma) is interpreted as being due to thermal emission from intracluster dust distributed in the hot X-ray emitting intracluster medium. The integrated excess flux within the central region of 10 ′–15 ′ diameter is ≈ 2.8 Jy. Since the dust temperature is poorly constrained, only a rough estimate of the associated dust mass of ${M_{\rm D} \approx 10^{7}\,M_{\odot}}$ can be derived. The associated visual extinction is negligible ($A_{V} \ll 0.1 ~{\rm mag}$) and much smaller than claimed from optical observations. No evidence is found for intracluster dust in the other five clusters observed. The absence of any signature for intracluster dust in five clusters and the rather low inferred dust mass in Abell 1656 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 in other wavelengths regions. 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.