Context : We present one of the first resolved maps of the [ CII ] 158 \mu m 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 about 1 kpc in size near the quasar nucleus , and we infer a star formation surface density \gtrsim 150 M _ { \odot } ~ { } yr ^ { -1 } ~ { } kpc ^ { -2 } , similar to that observed in local ULIRGs .
We also reveal another [ CII ] component , extended over \sim 12 kpc , and located at about 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 .

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Conclusions : Galaxies : high redshift – Galaxies : ISM – quasars : individual : BRI 0952-0115 – Submillimeter – Infrared : galaxies