2002MNRAS.337L...6T

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 ρ∝r ^–γ' with γ'= 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: H₀= 59 ⁺¹²_–7± 3 km s ^–1 Mpc ^–1 (68 per cent confidence level; Ω_m= 0.3, Ω _Λ= 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.