  The nearby A4-type star Fomalhaut hosts a debris belt in the form of an
eccentric ring, which is thought to be caused by dynamical influence from a
giant planet companion. In 2008, a detection of a point-source inside the inner
edge of the ring was reported and was interpreted as a direct image of the
planet, named Fomalhaut b. The detection was made at ~600--800 nm, but no
corresponding signatures were found in the near-infrared range, where the bulk
emission of such a planet should be expected. Here we present deep observations
of Fomalhaut with Spitzer/IRAC at 4.5 um, using a novel PSF subtraction
technique based on ADI and LOCI, in order to substantially improve the Spitzer
contrast at small separations. The results provide more than an order of
magnitude improvement in the upper flux limit of Fomalhaut b and exclude the
possibility that any flux from a giant planet surface contributes to the
observed flux at visible wavelengths. This renders any direct connection
between the observed light source and the dynamically inferred giant planet
highly unlikely. We discuss several possible interpretations of the total body
of observations of the Fomalhaut system, and find that the interpretation that
best matches the available data for the observed source is scattered light from
transient or semi-transient dust cloud.
