In some systems dominated by cold dust emission there is evidence that the cold dust emission is external to any optically emitting region and/or that dust is supplied from an external dust reservoir.
Thus, the unexpected result that large amounts of cold dust exist in some Virgo BCDs (Popescu et al. 2002) was interpreted as being indicative of dust surrounding the optical galaxy, originating in an external dust reservoir. In fact, in two cases direct evidence was found of resolved emission at 170 micron on scales of up to 10 kpc. The BCD galaxies were found to have the highest dust mass surface densities (normalised to optical area) and the coldest dust temperatures of the galaxies in the sample. This is a particularly unexpected result, since the IRAS observations of BCDs could be accounted for in terms of dust heated locally in HII regions, with temperatures of 30K or more. To qualitatively account for the FIR and optical extinction characteristics of BCDs, Popescu et al. (2002) proposed two scenarios invoking collisionally or photon-heated emission from grains originating in the surrounding intergalactic medium. In the one scenario, grains are swept up from a surrounding protogalactic cloud and heated collisionally in an optically thin wind bubble blown from the BCD. In the other, the grains are taken to be photon-heated in an optically thick disk surrounding the optical galaxy. The disk is indicative of a massive gas/dust accreting phase which makes dwarf galaxies sporadically bright optical-UV sources when viewed out of the equatorial plane of the disk. In both scenarios the dust does not have a galactic origin, but needs to exist in the immediate vicinity of the galaxies, where it can either be heated by winds or can accrete into the dwarfs.
Dust outside galaxies has also been discovered in a deep ISOPHOT survey of a field centred on the giant elliptical galaxy M86 in the Virgo cluster by Stickel et al. (2002b). One of the sources of FIR emission seen in the periphery of the field is extremely cold and has no obvious optical counterpart. It could trace a dust-rich "relic" of the interstellar media of two spiral galaxies removed in an interaction as postulated by Völk & Xu (1994). Such objects could themselves undergo localised episodes of star formation and could conceivably account for the large dust masses associated with the Virgo BCDs.
Moving more in the direction of the intragroup medium, the classical example is Stephan's Quintet (SQ), mapped by ISOPHOT using its oversampling mapping mode P32. The 60µm map (Sulentic et al. 2001) and the 100µm map (Xu & Tuffs 2002) show the probable detection of FIR diffuse emission from the intragroup medium. In particular the 100µm map shows clear evidence for extended FIR emission in the periphery of SQ, its morphology having a striking resemblance to the morphology of the diffuse syncrotron radio emission. Since the diffuse radio emission very probably traces a large scale shock in the intragroup medium, the diffuse FIR emission is likely to be also associated with the passage of the shock front. Such an association was predicted by Popescu et al. (2000b) for the case of large-scale accretion shocks in clusters.
Acknowledgments
We thank Dr. Jörg Fischera for providing us with Fig. 1, and Prof. Heinrich Völk, for informative discussions.