An Estimate ofH0from Keck Spectroscopy of the Gravitational Lens System 0957+561

We present long-slit LRIS/Keck spectroscopic observations of the gravitational lens system 0957]561. Averaged over all of our data, the rest-frame velocity dispersion of the central lens galaxy G1 is p v p v \ 279 ¹2 km s~1. However, there appears to be a signicant decrease in as a function of distance p v from the center of G1 that is not typical of the brightest cluster galaxies. Within of the center of G1, 0A. 2 we nd the average km s~1, whereas for positions greater than from the center of G1, p v \ 316 ¹4 0A. 2 we nd the average km s~1. A plausible explanation is that G1 contains a central massive p v \ 266 ¹2 dark object of mass km s~1 Mpc~1), which contributes to the M MDO B 4 ] 109 h 100 ~1 M _ (h 100 \ H 0 /100 central velocity dispersion, and that the outer value of is the appropriate measure of the depth of the p v potential well of G1. The determination of a luminosity-weighted estimate of is essential for a determi-p v nation of from Q0957]561 ; our accurate measurements remove one of the chief uncertainties in the H 0 assumed form of the mass distribution of the lens. Thus, with the recent apparent reduction in the uncertainty in the measurement of the time delay for the images A and B of Q0957]561, *q BA \ 417 ³ days (Kundic et al. ), we obtain an estimate for the Hubble constant: km s~1 Mpc~1. If for H 0 \ 62 ⁷ some reason the trend of