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

Abstract

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 (RE ≈ 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; Ωm = 0.3, ΩΛ = 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.