Spectroscopy of the lens galaxy of Q0957+561A,B: Implications of a possible central massive dark object

We present new long-slit William Herschel Telescope spectroscopic observations of the lens galaxy G1 associated with the double-imaged QSO 0957+561A,B. The central stellar velocity dispersion obtained, sigma(l) = 310 +/- 20 km s(-1), is in reasonable agreement with other measurements of this dynamical parameter. Using all updated measurements of the stellar velocity dispersion in the internal region of the galaxy (at angular separations <1 ".5) and a simple isotropic model, we discuss the mass of a possible central massive dark object. We find that the data of Falco et al. suggest the existence of an extremely massive object of (0.5-2.1) x 10(10) h(-1) M. (80% confidence level), whereas the inclusion of very recent data substantially changes the results: the compact central mass must be less than or equal to 6 x 10(9) h(-1) M. at the 90% confidence level. We note that, taking into account all the available dynamical data, a compact nucleus with a mass of 109 h(-1) M. (best fit) cannot be ruled out.