title: Intergalactic cold dust in the NGC 4631 group.
abstract: We have detected extraplanar cold dust at distances out to >10 kiloparsecs, situated in the halo of the interacting galaxy NGC 4631. The dust emission disk is much thinner than the warped HI disk, and new structures emerge. In particular, a giant arc has been found that is linked to anomalies in the kinematical structure of the atomic gas. Most of the extraplanar dust is closely associated with HI spurs that have been found earlier [Weliachew, L., Sancisi, R. & Guelin, M. (1978) Astron. Astrophys. 65, 37-45; Rand, R. J. (1994) Astron. Astrophys. 285, 833-856]. These spurs obviously are traces of the interaction [Combes, F. (1978) Astron. Astrophys. 65, 47-55]. The dust emission within the plane reaches the border of the optical disk. The activity of the disk of NGC 4631 is moderately enhanced by the interaction, but no gas moving in the z-direction could be found [Rand, R. J., Kulkarni, S. R. & Hester, J. J. (1992) Astrophys. J. 396, 97-103; Golla, G., Dettmar, R.-J. & Domgorgen, H. (1996) Astron. Astrophys. 313, 439-447]. Hence, it seems unlikely that strong winds have deposited the high-z dust. Instead, the coincidence with the HI features suggests that we see a track left behind by the interaction. In addition, the HI shows a supershell formed by an impact [Rand, R. J. & Stone, J. M. (1996) Astron. J. 111, 190-196] in the zone where the dust trail crosses the disk. This region is also characterized by disturbances in the distribution of the Halpha light. The masses associated with the dust can be estimated only very roughly on the basis of the existing data; they are of the order of a few 10(9) M middle dot in circle of gas.