A gas-rich AGN near the centre of a galaxy cluster at z ~ 1.4^⋆,⋆⋆

¹ INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy
e-mail: casasola@ira.inaf.it
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

Abstract

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.