2007A&A...470...53A

Gravitational lenses offer unique opportunities to explore the molecular content around active galactic nuclei at high redshift, through the magnification and gain in angular resolution. We study here the molecular and dust content of the high redshift (z=3.2), gravitationally lensed quasar MG0751+2716. We used the IRAM Plateau de Bure interferometer to measure the CO J=3-2, J=4-3 and J=8-7 rotation lines and continuum flux between 1.1mm and 3mm in MG0751+2716. We use HST data and constraints from the high resolution MERLIN map at 6.2 cm to build an improved model of the lens. The position agreement between the CO emission and the millimeter continuum suggests that dust and molecules are closely related. A 500 km/s velocity gradient is tentatively detected from the CO lines across the central 0.5''. The continuum SED in the range 0.45mm-204mm exhibits the signatures of two components: synchrotron emission from a core/jet system, and thermal emission related to warm dust. The CO line intensities are compatible with warm, dense molecular gas, suggesting a location in the close environment of the quasar. These observational constraints lead us to consider a source/quasar model made of a point-like core, a radio jet, and a molecular and dusty ring-like structure (∼460pc in radius) located in a plane perpendicular to the jet axis and seen almost edge-on. For this configuration, we derive a total magnification of 16 for the quasar radiation at rest frequency above 350 GHz and a magnification of 25 below 350 GHz. The dust mass (assuming a likely temperature T=50K) is M_dust=1.7x10⁸M_☉, the mass of molecular material is M(H₂)=6x10¹⁰M_☉ and a lower limit to the dynamical mass is M_dyn=1.5x10¹⁰M_☉. Such values are comparable to those found in the small sample of CO detected sources at high redshifts.