BR1202–0725: an extreme multiple merger at z = 4.7^⋆

¹ LERMA, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: philippe.salome@obspm.fr
² Institut de Radio Astronomie Millimétrique (IRAM), Domaine Universitaire, 300 rue de la piscine, 38406 St Martin d’Hères, France
³ LAB, Observatoire de Bordeaux, 2 rue de l’Observatoire, BP 89, 33270 Floirac, France
⁴ IAP, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France

Received: 5 July 2012
Accepted: 27 July 2012

Abstract

The radio-quiet quasar BR1202–0725 (z = 4.695) is a remarkable source with a bright northwest (NW) companion detected at submillimeter and radio wavelengths but invisible in the optical. In the absence of amplification by gravitational lensing, BR1202−0725 would be the most luminous binary CO and far-infrared (far-IR) source in the Universe. In this paper, we report observations with the IRAM Plateau de Bure interferometer of BR1202–0725 in the redshifted emission of the CO(5–4) and (7–6) lines, the [CI](³P₂-³P₁) line, a high angular resolution (0.3″ × 0.8″) 1.3 mm map of the rest-frame, far-IR dust continuum, and a search for the CO(11−10) line. We compare these results with recent ALMA data in the [CII] line. Both the quasar host galaxy and its NW companion are spatially resolved in the molecular line emission and the dust continuum. The CO profile of the NW companion is very broad with a full width at half maximum of ~1000 ± 130 km s^-1, compared to ~360 ± 40 km s^-1 for the quasar host galaxy to the southeast (SE). The difference in linewidths and center velocities, and the absence of any lens candidate or arc-like structure in the field, at any wavelength, show that the obscured NW galaxy and the SE quasar host galaxy cannot be lensed images of the same object. Instead, we find morphological and kinematic evidence for sub-structures in both the NW and SE sources. We interpret these results as strong indications that the BR1202–0725 complex is a group of young, interacting, and highly active starburst galaxies.