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.