Strong lensing by a node of the cosmic web

The core of MACS J0717.5+3745 at z = 0.55^⋆

¹ Laboratoire d’Astrophysique de Marseille, UMR 6610, CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13 388 Marseille Cedex 13, France
e-mail: marceau.limousin@oamp.fr
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr, Honolulu, HI 96822, USA
⁴ CRAL, Observatoire de Lyon, Université Lyon 1, 9 Avenue Ch. André, 69561 Saint Genis Laval Cedex, France
⁵ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁶ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁷ Department of Astronomy, California Institute of Technology, 105-24, Pasadena, CA 91125, USA
⁸ Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, University of KwaZulu-Natal, 4041 Durban, South Africa
⁹ Department of Physics & Astronomy, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, N2L 3G1, Canada
¹⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts, 02138-1516, USA
¹¹ Jet Propulsion Laboratory, Caltech, MS 169-327, 4 800 Oak Grove Dr, Pasadena, CA 91109, USA

Received: 20 August 2011
Accepted: 4 June 2012

Abstract

We present results of a strong-lensing analysis of MACS J0717.5+3745 (hereafter MACS J0717), an extremely X-ray luminous galaxy cluster at z = 0.55. Observations at different wavelengths reveal a complex and dynamically very active cluster, whose core is connected to a large scale filament extended over several Mpc. Using multi-passband imaging data obtained with the Hubble Space Telescope’s Advanced Camera for Surveys (ACS), we identify 15 multiply imaged systems across the full field of view of ACS, five of which we confirmed spectroscopically in ground-based follow-up observations with the Keck telescope. We use these multiply imaged systems to constrain a parametric model of the mass distribution in the cluster core, employing a new parallelized version of the Lenstool software. The main result is that the most probable description of the mass distribution comprises four cluster-scale dark matter haloes. The total mass distribution follows the light distribution but strongly deviates from the distribution of the intra-cluster gas as traced by the X-ray surface brightness. This confirms the complex morphology proposed by previous studies. We interpret this segregation of collisional and collisionless matter as strong evidence of multiple mergers and ongoing dynamical activity. MACS J0717 thus constitutes one of the most disturbed clusters presently known and, featuring a projected mass within the ACS field of view (R = 150″ = 960 kpc) of 2.11  ±  0.23  ×  10¹⁵   M_⊙, the system is also one of the most massive known.