## Abstract

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,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.

Original language | English |
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Pages (from-to) | 1969-1981 |

Number of pages | 13 |

Journal | Monthly Notices of the Royal Astronomical Society |

Volume | 408 |

Issue number | 4 |

DOIs | |

Publication status | Published - 11-Nov-2010 |

## Keywords

- gravitational lensing: strong
- galaxies: structure
- EARLY-TYPE GALAXIES
- LENS ACS SURVEY
- SMALL-SCALE STRUCTURE
- MATTER SUBSTRUCTURE
- ELLIPTIC GALAXIES
- MASS SUBSTRUCTURE
- MILKY-WAY
- IDENTIFYING LENSES
- COSMOLOGICAL MODEL
- EVOLUTION