Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 408, Issue 4, Pages 1969-1981Publisher
OXFORD UNIV PRESS
DOI: 10.1111/j.1365-2966.2010.16865.x
Keywords
gravitational lensing: strong; galaxies: structure
Categories
Funding
- NWO-VIDI [639.042.505]
- NSF [NSF-0642621]
- Sloan Foundation
- Packard Foundation
- NASA [SNAP-10174, GO-10494, SNAP-10587, GO-10798, GO-10886, NAS 5-26555]
- Alfred P. Sloan Foundation
- National Aeronautics and Space Administration
- National Science Foundation
- US Department of Energy
- Japanese Monbukagakusho
- Max Planck Society
- University of Chicago, Fermilab
- Institute for Advanced Study
- Japan Participation Group
- Johns Hopkins University
- Korean Scientist Group
- Los Alamos National Laboratory
- Max-Planck-Institute for Astronomy (MPIA)
- Max-Planck-Institute for Astrophysics (MPA)
- New Mexico State University
- University of Pittsburgh
- University of Portsmouth
- Princeton University
- United States Naval Observatory
- University of Washington
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We report the detection of a dark substructure - undetected in the Hubble Space Telescope HST ACS F814W image -in the gravitational lens galaxy SDSSJ0946+1006 (the 'double Einstein ring'), through direct gravitational imaging. The detection of a small mass concentration in the surface density maps, at 4.3 kpc from the galaxy centre, has a strong statistical significance. We confirm this detection by modelling the substructure with a tidally truncated pseudo-Jaffe density profile; in that case the substructure mass is M-sub = (3.51 +/- 0.15) x 10(9) M-circle dot, precisely where also the surface density map shows a strong convergence peak (Bayes factor Delta log epsilon = -128.0; equivalent to a similar to 16 sigma detection). The result is robust under substantial changes in the model. We set a lower limit of (M/L)(V,circle dot) greater than or similar to 120 M-circle dot/L-V,L-circle dot (3 sigma) inside a sphere of 0.3 kpc centred on the substructure (r(tidal) = 1.1 kpc). The mass and luminosity limit of this substructure are consistent with Local Group results if the substructure had a virial mass of similar to 10(10) M-circle dot before accretion and formed at z greater than or similar to 10. Our detection implies a projected dark matter mass fraction in substructure at the radius of the inner Einstein ring of f = 2.15(-1.25)(+2.05) per cent [68 per cent confidence level (CL)] in the mass range 4 x 10(6)-4 x 10(9) M-circle dot, assuming alpha = 1.9 +/- 0.1 (with dN/dm alpha m(-alpha)). Assuming a flat prior on alpha, between 1.0 and 3.0, increases this to f = 2.56(-1.50)(+3.26) per cent (68 per cent CL). The likelihood ratio is similar to 0.5 between these fractions and that from simulations (f(N-body) approximate to 0.003). Hence the inferred dark matter mass fraction in substructure, admittedly based on a single-lens system, is large but still consistent with predictions.
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