The internal structure of the lens PG1115+080: breaking degeneracies in the value of the Hubble constant

We combine lensing, stellar kinematic and mass-to-light ratio constraints to build a twocomponent (luminous plus dark) mass model of the early-type lens galaxy in PG1115+080. We find a total mass density profile steeper than r -2 , effectively r - with = 2.35 0.1 0.05 (random + systematic). The stellar mass fraction is f * = 0.67 +0.20 -0.25 0.03 inside the Einstein radius (R E 1.2 effective radii). The dynamical mass model breaks the degeneracies in the mass profile of the lens galaxy and allows us to obtain a value of the Hubble constant that is no longer dominated by systematic errors: H 0 = 59 +12 -7 3 km s -1 Mpc -1 (68 per cent confidence level; m = 0.3, = 0.7). The offset of PG1115+080 from the Fundamental Plane might indicate deviations from homology of the mass profile of some early-type galaxies.

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