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Astron. Astrophys. 329, 827-839 (1998)

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Modelling the Cloverleaf: contribution of a galaxy cluster at z ~ 1.7*

J.-P. Kneib 1, D. Alloin 2, Y. Mellier 3, 4, 1, S. Guilloteau 5, R. Barvainis 6 and R. Antonucci 7

1 Observatoire Midi-Pyrénées, CNRS-UMR5572, 14 Av. Edouard Belin, F-31400 Toulouse, France
2 CNRS-URA2052, Service d'Astrophysique, CE Saclay, l'Orme des Merisiers, F-91191 Gif-sur -Yvette Cedex, France
3 Institut d'Astrophysique, 98 bis Bd. Arago, F-75014 Paris, France
4 Observatoire de Paris, DEMIRM, 61 Av. de l'Observatoire, F-75014 Paris, France
5 Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France
6 MIT Haystack Observatory, Westford, MA 01886, USA
7 UC Santa Barbara, Physics Department, Santa Barbara, CA 93106, USA

Received 6 June 1997 / Accepted 26 June 1997


We present a new investigation of the Cloverleaf (z [FORMULA] 2.558) based on the combination of archival HST/WFPC2 data, recent IRAM CO(7-6) maps and wide field CFHT/FOCAM images. The deepest WFPC2 observation (F814W) shows a significant overdensity of I814W ~ 23-25 galaxies around the Cloverleaf that we interpret as the presence of a cluster of galaxies along the line of sight. The typical magnitude, red color (R-I [FORMULA] 0.9) and small angular size of these galaxies suggest that the cluster is very distant and could be associated with the absorption systems observed in the spectra of the quasar spots (either z [FORMULA] 1.438, 1.66, 1.87 or 2.07). The Cloverleaf is probably the result of the lensing effects of a system which includes, in addition to a single galaxy, one of the most distant clusters of galaxies ever detected.

With this assumption, we have modelled the lens using the HST/WFPC2 data and the IRAM/CO(7-6) map. We have considered two cases: one in which the mass model is a galaxy and a dark halo at z = 1.7, and a second one in which the mass model is the combination of a cluster (centered on the overdensity of galaxies) and an individual galaxy located amid the Cloverleaf, both at z = 1.7. The high-resolution IRAM/CO map provides for the first time the orientation and the ellipticity of the CO spots induced by the shear component. Velocity - positional effects are detected at the 8 [FORMULA] level in the CO map. A strong limit can then be put on the size, shape and location of the CO source around the quasar. The CO source is found to form a disk- or ring-like structure orbiting the central engine at [FORMULA] 100 km/s at a radial distance of [FORMULA] 100 pc, leading to a central mass of [FORMULA] 109 M [FORMULA] possibly in the form of a black hole. The cluster component increases significantly the convergence of the lens and this pulls down the requirement on the mass of the lensing galaxy by a factor 2. This may help explain the mystery of why the lensing galaxy has not been detected yet. A deep high resolution infrared image should reveal the nature and location of the lensing galaxy.

The presence of an additional lensing cluster along the line of sight to the Cloverleaf strengthens the suspicion that many bright quasars are magnified by distant clusters of galaxies at redshifts larger than 1.

Key words: gravitational lensing – clusters of galaxies – gravitational lensing – quasars: Cloverleaf (H1413+117) – galaxies: ISM

* Based on observations obtained at the Institut de Radio Astronomie Millimétrique, supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain), at the Canada France Hawaii Telescope, supported by INSU/CNRS, the CNRC (Canada) and the University of Hawaii, and with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.

Send offprint requests to: D. Alloin alloin@discovery.saclay.cea.fr

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997