BR 1202-0725 system in [CII] line

The most prominent sources of this system are a quasar (QSO) and a submillimeter galaxy (SMG), separated by a projected distance of about ~24kpc and characterized by very high star formation rates, higher than 1000M_{sun}_/yr. However, the ALMA observations reveal that these galaxies apparently have undisturbed rotating disks, which is at variance with the commonly accepted scenario in which strong star formation activity is induced by a major merger.We also detected faint components which, after spectral deblending, were spatially resolved from the main QSO and SMG emissions.

The relative velocities and positions of these components are compatible with orbital motions within the gravitational potentials generated by the QSO host galaxy and the SMG, suggesting that they are smaller galaxies in interaction or gas clouds in accretion flows of tidal streams. Moreover, we did not find any clear spectral evidence for outflows caused by active galactic nuclei or stellar feedback. This suggests that the high star formation rates might be induced by interactions or minor mergers with these companions, which do not affect the large-scale kinematics of the disks, however.