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Abstract: We report the detection of a dark substructure through direct gravitationalimaging - undetected in the HST-ACS F814W image - in the gravitational lensgalaxy of SLACS SDSSJ0946+1006 the -Double Einstein Ring-. The detection isbased on a Bayesian grid reconstruction of the two-dimensional surface densityof the galaxy inside an annulus around its Einstein radius few kpc.

. Weconfirm this detection by modeling the system including a parametric mass modelwith a tidally truncated pseudo-Jaffe density profile; in that case thesubstructure mass is M sub=3.51+-0.15x10^9 Msun, located at-0.651+-0.038,1.040+-0.034-, precisely where also the surface density mapshows a strong convergence peak.

. We set a lower limit of M-L V}>=120Msun-L} V,sun 3-sigma inside a sphere of 0.3 kpc centred on the substructurer tidal=1.1kpc. The result is robust under substantial changes in the modeland the data-set e.g. PSF, pixel number and scale, source and potentialregularization, rotations and galaxy subtraction. Despite being at the limitsof detectability, it can therefore not be attributed to obvious systematiceffects. Our detection implies a dark matter mass fraction at the radius of theinner Einstein ring of f CDM=2.15^{+2.05} {-1.25} percent 68 percent C.L inthe mass range 4x10^6 Msun to 4x10^9 Msun assuming alpha=1.9+-0.1 with dN-dm ~m^-alpha. Assuming a flat prior on alpha, between 1.0 and 3.0, increases thisto f CDM=2.56^{+3.26} {-1.50} percent 68 percent C.L. The likelihood ratio is0.51 between our best value f CDM=0.0215 and that from simulationsf sim=0.003. Hence the inferred mass fraction, admittedly based on a singlelens system, is large but still consistent with predictions.


Autor: S. Vegetti, L.V.E. Koopmans Kapteyn Astronomical Institute, A. Bolton IfA-Utah, T. Treu UCSB, R. Gavazzi IAP


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