Dust in the Intergalactic Medium of Galaxy Clusters

Far-infrared (FIR) observations with ISOPHOT aboard the Infrared Space Observatory were carried out to observe the thermal FIR emission from diffuse intracluster dust (ICD) in galaxy clusters and outlying dust structures in the elliptical galaxies M86 and NGC5128 (Centaurus A). Extended FIR emission from ICD has only been detected in the Coma cluster. It is likely caused by tidal interactions, since Coma is a dynamically young cluster with on-going gravitational interactions in the centre and infalling galaxy groups. M86 has a complex FIR morphology with a compact double source near its centre and several off-centre sources. The brightest FIR source lies between M86 and the nearby spiral NGC 4402 and represents the first direct detection of a localized intracluster dust cloud not associated with optical or neutral hydrogen emission. Overall, the FIR data do not support the ram pressure dust stripping scenario suggested on the basis of IRAS data, but indicate a significant influence of tidal interactions. NGC 5128 shows obvious optical signs of interaction, most notably a central dust lane and stellar shells. The FIR emission of outlying dust 15 kpc from the centre has been discovered where previously HI and molecular gas closely associated with the shells had been found. NCC 5128 is the first case of a shell galaxy where all components of the interstellar medium (ISM) are detected far off the nucleus, indicating an inner disk origin of the outlying material.