Context : The formation of the first virialized structures in overdensities dates back to \sim 9 Gyr ago , i.e .
in the redshift range z \sim 1.4 - 1.6 .
Some models of structure formation predict that the star formation activity in clusters was high at that epoch , implying large reservoirs of cold molecular gas .

Aims : Aiming at finding a trace of this expected high molecular gas content in primeval clusters , we searched for the ^ { 12 } CO ( 2–1 ) line emission in the most luminous active galactic nucleus ( AGN ) of the cluster around the radio galaxy 7C 1756+6520 at z \sim 1.4 , one of the farthest spectroscopic confirmed clusters .
This AGN , called AGN.1317 , is located in the neighbourhood of the central radio galaxy at a projected distance of \sim 780 kpc .

Methods : The IRAM Plateau de Bure Interferometer was used to investigate the molecular gas quantity in AGN.1317 , observing the ^ { 12 } CO ( 2–1 ) emission line .

Results : We detect CO emission in an AGN belonging to a galaxy cluster at z \sim 1.4 .
We measured a molecular gas mass of 1.1 \times 10 ^ { 10 } M _ { \odot } , comparable to that found in submillimeter galaxies .
In optical images , AGN.1317 does not seem to be part of a galaxy interaction or merger .
We also derived the nearly instantaneous star formation rate ( SFR ) from H \alpha flux obtaining a SFR \sim 65 M _ { \odot } yr ^ { -1 } .
This suggests that AGN.1317 is actively forming stars and will exhaust its reservoir of cold gas in \sim 0.2-1.0 Gyr .

Conclusions :