The first detection of Far-Infrared emission associated with an extended HI disk. The case of NGC 891

Spiral galaxies in the local universe are commonly observed to be embedded in extended disks of neutral hydro- gen - the so called "extended HI disks". Based on observations made using the ISOPHOT instrument on board the Infrared Space Observatory, we report the first detection of cold dust in the extended HI disk of a spiral galaxy. The detection was achieved through a dedicated deep Far-Infrared observation of a large field encompassing the entire HI disk of the edge-on spiral galaxy NGC 891. Our discovery indicates that the extended HI disk of NGC 891 is not primordial in origin.