Estimates of the Hubble Constant

To estimate the value of H₀ from these time delays requires a good model of the deflector potentials. In all three GL systems, it is assumed that these are dominated by the potential of the primary lens galaxies (two in the case of B1608+656) and that these galaxies have an isothermal mass distribution.

Under these assumptions (see the references for more details) one finds: H₀=

69⁺¹³_-19б kms^-1Mpc^-1 (95%) from B0218+357 (Biggs et al. 1999), H₀=

60⁺¹⁵_-12б kms^-1Mpc^-1 (95%) from B1600+434 (Koopmans et al. 2000) and H₀=63⁺⁷_-6б kms^-1Mpc^-1 (95%) from B1608+656 (Koopmans & Fassnacht 1999) with

=0.3 and

=0.7. Burud et al. (2000) estimate a slightly lower value of H₀ from B1600+434, using the mass models from Maller et al. (2000). In addition, Lehar et al. (2000) claim a larger systematic error on the value of H₀ from B0218+357, due to the uncertainty in the position of the lens galaxy. Results from modeling the Einstein ring in B0218+357 (Wucknitz, private communications) seem to agree with the galaxy position used by Biggs et al. (1999), however. Although we stress that these are preliminary values, the interesting conclusion from a comparison of these values of H₀, is their good agreement not only with determinations from other GL systems, but also with those from the HST Key-Project, S-Z measurements and determinations from high-redshift SNe Ia (e.g. Koopmans & Fassnacht 1999; Freedman et al. 2001).

Even so, the uncertainties are still considerable and not all possible mass models have been fully explored yet. In none of these cases for example does the error include the uncertainty in the slope of the radial mass profile or the center of the mass distribution, which dominate the systematic uncertainties in the value of H₀. To improve this situation, Wucknitz et al. (in prep.) are using the additional information in the structure of the radio Einstein ring in B0218+357 to constrain the position of the lens galaxy and its radial mass profile. Similarly, Surpi & Blandford (in prep.) are using the arcs in B1608+656 to further constrain its mass distribution, whereas Fassnacht et al. (in prep.) have obtained data to measure the central velocity dispersion of the primary lens galaxy. In the case of B1600+434, no clear extended source structure is present, although Keck observations will be done to try to measure the velocity dispersion and rotation velocity of the bulge and disk, respectively.

б© Copyright Astronomical Society of Australia 1997