Resolved [CII] emission in a lensed quasar at z = 4.4

We present one of the first resolved maps of the [CII] 158 µm line, a powerful tracer of the star forming inter-stellar m edium, 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 dist ributed over a region of about 1 kpc in size near the quasar nucleus, and we infer a star formation surface density& 150 M⊙ yr −1 kpc −2 , similar to that observed in local ULIRGs. We also reveal another [CII] component, extended over∼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 quasa r 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.