A Determination of H0 with the CLASS Gravitational Lens B1608+656. II. Mass Models and the Hubble Constant from Lensing

We present mass models of the four-image gravitational lens system B1608+656, based on information obtained through VLBA imaging, VLA monitoring, and Hubble Space Telescope (HST) WFPC2 and NICMOS imaging. We have determined a mass model for the lens galaxies that reproduces (1) all image positions within the observational errors, (2) two out of three flux-density ratios within about 10% from the observed ratios, and (3) the model time delays within 1% from their observed values, given our best estimate of the Hubble parameter. Using the time delays determined in a companion paper, we also find that the best isothermal mass model gives H₀=59⁺⁷_-6 km s^-1 Mpc^-1 for Ω_m=1 and Ω_Λ=0.0, or H₀=(65-63)⁺⁷_-6 km s^-1 Mpc^-1 for Ω_m=0.3 and Ω_Λ=0.0-0.7. The statistical errors indicate the 95.4% (2 σ) confidence interval. A systematic error of +/-15 km s^-1 Mpc^-1 is estimated from a 20% (1 σ) uncertainty in the steepness of radial mass profile. This cosmological determination of H₀ agrees well with determinations from three other gravitational lens systems (i.e., B0218+357, Q0957+561, and PKS 1830-211), Type Ia supernovae, the Sunyaev-Zeldovich effect and local determinations. The current agreement on H₀--within the 1 σ statistical errors--from four of five gravitational lens systems (1) emphasizes the reliability of its determination from isolated gravitational lens systems and (2) suggests that a close-to-isothermal mass profile can describe disk galaxies (e.g., B0218+357 and possibly PKS 1830-211), ellipticals (e.g., B1608+656), and central cluster ellipticals (e.g., Q0957+561). The average of H₀ from B0218+357, Q0957+561, B1608+656, and PKS 1830-211, gives H^GL₀=69+/-7 km s^-1 Mpc^-1 for a flat universe with Ω_m=1 or H^GL₀=74+/-8 km s^-1 Mpc^-1 for Ω_m=0.3 and Ω_Λ=0.0-0.7. When including PG 1115+080, these values decrease to 64+/-11 km s^-1 Mpc^-1 and 68+/-13 km s^-1 Mpc^-1, respectively. The errors are the estimated 2 σ errors on the average. The Hubble parameter from gravitational lenses seems to agree best with local determinations of H₀ for a low-density universe, under the assumption that all lenses are nearly isothermal.