A Determination ofH0with the CLASS Gravitational Lens B1608+656. II. Mass Models and the Hubble Constant from Lensing

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Abstract
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 km s-1 Mpc-1 for Ωm = 1 and ΩΛ = 0.0, or H0 = (65-63) km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 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 = 69 ± 7 km s-1 Mpc-1 for a flat universe with Ωm = 1 or H = 74 ± 8 km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 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.
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