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Strongly star-forming rotating disks in a complex merging system at z = 4,7 as revealed by ALMA

We performed a kinematical analysis of the [CII] line emission of the BR 1202-0725 system at z~4,7 using ALMA observations. The most prominent sources of this system are a quasar and a submillimeter galaxy, separated by a projected distance of about 24 kpc and characterized by very high SFR, higher than 1000 Msun/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. We did not find any clear spectral evidence for outflows caused by AGN 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. Our kinematical analysis also indicates that the QSO and the SMG have similar Mdyn, mostly in the form of molecular gas, and that the QSO host galaxy and the SMG are seen close to face-on with slightly different disk inclinations: the QSO host galaxy is seen almost face-on (i~15), while the SMG is seen at higher inclinations (i~25). Finally, the ratio between the black hole mass of the QSO, obtained from XShooter spectroscopy, and the Mdyn of the host galaxy is similar to value found in very massive local galaxies, suggesting that the evolution of black hole galaxy relations is probably better studied with dynamical than with stellar host galaxy masses.