Radio counterpart of the lensed submm emission in the cluster MS0451.6-0305: new evidence for the merger scenario

Context. SMM J04542-0301 is an extended (similar to 1') submm source located near the core of the cluster MS0451.6-0305. It has been suggested that part of its emission arises from the interaction between a LBG and two EROs at z similar to 2.9 that are multiply-imaged in the optical/NIR observations. However, the dramatic resolution difference between the sub-mm map and the optical/NIR images make it di. cult to confirm this hypothesis. Aims. In a previous paper, we reported the detection of 1.4 GHz continuum radio emission coincident with this sub-mm source using VLA archival data. To fully understand the relation between this radio emission, the sub-mm emission, and the optical/IR multiplyimaged sources, we have re-observed the cluster with the VLA at higher resolution. Methods. The previous archival data has been re-reduced and combined with the new observations to produced a deep (similar to 10 mu Jy beam(-1)), high resolution (similar to 2 '') map centred on the cluster core. The strong lensing effect in the radio data has been quantified by constructing a new lens model of the cluster. Results. From the high resolution map we have robustly identified six radio sources located within SMM J 04542-0301. The brightest and most extended of these sources (RJ) is located in the middle of the sub-mm emission, and has no obvious counterpart in the optical/NIR. Three other detections (E1, E2 and E3) seem to be associated with the images of one of the EROs (B), although the NIR and radio emission appear to originate at slightly different positions in the source plane. The last two detections (CR1 and CR2), for which no optical/ NIR counterpart have been found, seem to constitute two relatively compact emitting regions embedded in a similar to 5 '' extended radio source located at the position of the sub-mm peak. The presence of this extended component (which contributes 38% of the total radio flux in this region) can only be explained if it is being produced by a lensed region of dust obscured star formation in the center of the merger. A comparison between the radio and sub-mm data at the same resolution suggests that E1, E2, E3, CR1 and CR2 are associated with the sub-mm emission. Conclusions. The radio observations presented in this paper provide strong observational evidence in favour of the merger hypothesis. However, the question if RJ is also contributing to the observed sub-mm emission remains open. These results illustrate the promising prospects for radio interferometry and strong gravitational lensing to study the internal structure of SMGs.