Volume 558, October 2013
|Number of page(s)||5|
|Published online||03 October 2013|
1 INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy
2 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3 Observatoire de Paris, LERMA, 61 Av. de l’Observatoire, 75014 Paris, France
4 Dipartimento di Fisica e Astronomia, Università di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
Received: 29 March 2013
Accepted: 12 July 2013
Context. The formation of the first virialized structures in overdensities dates back to ~9 Gyr ago, i.e. in the redshift range z ~ 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 12CO(2–1) line emission in the most luminous active galactic nucleus (AGN) of the cluster around the radio galaxy 7C 1756+6520 at z ~ 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 ~780 kpc.
Methods. The IRAM Plateau de Bure Interferometer was used to investigate the molecular gas quantity in AGN.1317, observing the 12CO(2−1) emission line.
Results. We detect CO emission in an AGN belonging to a galaxy cluster at z ~ 1.4. We measured a molecular gas mass of 1.1 × 1010M⊙, 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 Hα flux obtaining a SFR ~ 65 M⊙ yr-1. This suggests that AGN.1317 is actively forming stars and will exhaust its reservoir of cold gas in ~0.2−1.0 Gyr.
Key words: galaxies: active / galaxies: individual: AGN 1317 / galaxies: clusters: individual: 7C 1756+6520 / galaxies: distances and redshifts / galaxies: high-redshift / galaxies: ISM
Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
Reduced IRAM data is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/558/A60
© ESO, 2013
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