  We present one of the first resolved maps of the [CII] 158 micron line, a
powerful tracer of the star forming inter-stellar medium, at high redshift. We
use the new IRAM PdBI receivers at 350 GHz to map this line in BRI 0952-0115,
the host galaxy of a lensed quasar at z=4.4 previously found to be very bright
in [CII] emission. The [CII] emission is clearly resolved and our data allow us
to resolve two [CII] lensed images associated with the optical quasar images.
We find that the star formation, as traced by [CII], is distributed over a
region of ~ 1 kpc in size near the quasar nucleus, and we infer a star
formation surface density >150 Msun/yr/kpc^2, similar to that observed in local
ULIRGs. We also reveal another [CII] component, extended over ~ 12 kpc, and
located at ~ 10 kpc from the quasar. We suggest that this component is a
companion disk galaxy, in the process of merging with the quasar host, whose
rotation field is distorted by the interaction with the quasar host, and where
star formation, although intense, is more diffuse. These observations suggest
that galaxy merging at high-z can enhance star formation at the same time in
the form of more compact regions, in the vicinity of the accreting black hole,
and in more extended star forming galaxies.
