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: H_0= 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.

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