  The presence of dust in elliptical galaxies has recently been shown to be
quite common. Deep optical multi-colour CCD imaging has revealed the presence
of dust lanes and patches, and the technique of co-adding IRAS survey scans has
led to many detections of elliptical galaxies. The optical and far-infrared
surveys reported similar detection rates of dust, which may indicate that dust
in elliptical galaxies is generally distributed in the optically detected lanes
or patches. However, we show here that the amount of dust as derived from the
optical extinction values is typically an order of magnitude smaller than that
derived from the IRAS flux densities, in strong contrast with the situation in
spiral galaxies.
  To unriddle this dilemma, we postulate that the majority of the dust in
elliptical galaxies exists as a diffusely distributed component which is
undetectable using optical methods. Employing a multiple scattering model for
the dust, we show that the presence of this newly postulated dust component
implies significant radial colour gradients which were essentially thought to
arise from stellar population gradients only.
  The energetics of the diffusely distributed dust component is shown to be
consistent with the IRAS data. A comparison of the detectable amount of dust in
elliptical galaxies embedded in hot, X-ray-emitting gas with the amount
expected in case the production and destruction rates of dust are equal
indicates that most of the dust in elliptical galaxies has been accreted from
other galaxies.
