Dust and molecular content of the lensed quasar, MG0751+2716, at z = 3.2^*

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: danielle.alloin@cea.fr
² AIM, CEA/DSM-CNRS-Université Paris 7, SAp, Bât. 709, CE Saclay, l'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
³ Laboratoire d'Astrophysique de Marseille, OAMP Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
⁴ Laboratoire d'Astrophysique de Bordeaux, CNRS, OASU, Université Bordeaux 1, BP 89, 33270 Floirac, France
⁵ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France

Received: 26 September 2006
Accepted: 14 April 2007

Abstract

Context.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.

Aims.We study here the molecular and dust content of the high redshift (), gravitationally lensed quasar MG0751+2716.

Methods.We used the IRAM Plateau de Bure interferometer to measure the CO , and rotation lines and continuum flux between 1.1 mm and 3 mm 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.

Results.The position agreement between the CO emission and the millimeter continuum suggests that dust and molecules are closely related. A 500  velocity gradient is tentatively detected from the CO lines across the central 0.5″. The continuum SED in the range 0.45 mm–204 mm 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 (~460 pc 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  K) is  , the mass of molecular material is   and a lower limit to the dynamical mass is  . Such values are comparable to those found in the small sample of CO detected sources at high redshifts.