2010MNRAS.408.1969V

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)x10⁹M_☉, precisely where also the surface density map shows a strong convergence peak (Bayes factor; equivalent to a ∼16σ detection). The result is robust under substantial changes in the model. We set a lower limit of (M/L)_V,☉_ ≳ 120M_☉/L_V,☉_(3σ) inside a sphere of 0.3 kpc centred on the substructure (r_tidal= 1.1kpc). The mass and luminosity limit of this substructure are consistent with Local Group results if the substructure had a virial mass of ∼10¹⁰M_☉ before accretion and formed at z ≳ 10. Our detection implies a projected dark matter mass fraction in substructure at the radius of the inner Einstein ring of f = 2.15^+2.05_–1.25 per cent [68 per cent confidence level (CL)] in the mass range 4x10⁶- 4x10⁹M_☉, assuming α = 1.9±0.1 (with dN/dm ∝ m^–α). Assuming a flat prior on α, between 1.0 and 3.0, increases this to f = 2.56^+3.26_–1.50 per cent (68 per cent CL). The likelihood ratio is ∼0.5 between these fractions and that from simulations (f_N-body_~ 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.