The outermost parts of galaxies are among the most fascinating, yet
the less known regions of the Universe. Although the large optical/
radio nebulae have sizes of 30 kpc or so, circumgalactic matter is
observed at distances of 100 kpc from their nuclei. These extreme
regions could be the hideouts of dark matter; their slow revolution
and low stellar activity has kept them in many respects similar to
protogalactic clouds. Obviously, the difficulty of studying the outer
parts of galaxies comes from their faintness at all wavelengths. These
regions have very low densities and, in their densest parts, are very
cold. They are difficult to detect, except in absorption in the lines
of sight to bright quasars, or as diffuse sources of X-ray emission.
Progresses in their study relies on tracers which could be observed in
emission with a high angular resolution.
  Thermal emission from dust grains may be the best such tracer, and
ISO, with its exceptional sensitivity in the FIR, offers an unique
opportunity for this study. The detection of bright FIR, submm and mm
continuum emission in high redshift galaxies, and the detection of C,
O, and CO lines in some of these objects, show that heavy elements and
dust were abundant in the `primordial' gas and must be present in the
circumgalactic matter. Dust grains, in the vicinity of bright galaxies
are likely to have temperatures > 10 K; their thermal emission should
peak between 150 and 200 micro-m, beyond the IRAS spectral range, and
should be detectable by ISOPHOT.
  We propose to search for this emission with the PHOT-C200 detector
around a sample of nearby galaxies: interacting galaxies with HI
bridges and companions, isolated galaxies with and withoout HI warps,
edge-on galaxies with conspicious radio or optical halos.