2000ApJ...531..635M

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, σ_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)x10¹⁰ h^–1 M_☉ (80% confidence level), whereas the inclusion of very recent data substantially changes the results: the compact central mass must be ≤6x10⁹ 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.