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 (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.