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 two-component (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 rho proportional to r(-gamma)' with gamma' = 2.35 +/- 0.1 +/- 0.05 (random + systematic). The stellar mass fraction is f(*) = 0.67(-0.25)0.20 +/- 0.03 inside the Einstein radius (R-E approximate to 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(-7)(+12) +/- 3 km s(-1) Mpc(-1) (68 per cent confidence level; Omega(m) = 0.3, Omega(Lambda) = 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.