Old Q0957+561 data & software:
► Simple FORTRAN programs to apply the d2 test (accurate and robust time delay measurements)
► Photos
Advertising:
The system SBS 0909+532A,B is a double quasar with angular size of about 1 arcsec. Spectra taken in good seeing conditions with the 4.2 m William Herschel Telescope separated the A and B images, showing two components with the same redshift zs = 1.377 and identical spectra. The Mg II doublet was found in absorption at the same redshift (zl = 0.83) in both components A and B, although the equivalent width of the faintest one is 3 times stronger. From a simple lens model (singular isothermal sphere), assuming that the absorption system corresponds to the lensing galaxy, we derived an upper limit on the time delay of about 100 h-1 days (Oscoz et al. 1997, ApJ 491, L7).
We studied the stellar motion within the central region of a cD galaxy at redshift z = 0.36, which is the main lens galaxy associated with the double-imaged QSO 0957+561A,B. Previously, Falco et al. (1997, ApJ 484, 70) measured the 1D velocity dispersion associated with the luminous stars in this galaxy, reporting an apparent decrease with angular distance from the center (316 km/s at 0''.1 and 262 km/s at 0''.5). Any giant elliptical could have a central black hole of 107-109 solar masses, and so the gradient was interpreted by the authors as due to a central massive dark object of mass similar to the largest measured central mass in elliptical galaxies. This pioneering study on the velocity dispersion-angular distance relationship, the measurements by Tonry & Franx (1998, ApJ 515, 512) and the central stellar velocity dispersion obtained by our group with the William Herschel Telescope, are useful tools to a first serious discussion on the mass of a possible central massive dark object. Thus, using a simple isotropic model incorporating a compact central mass that is responsible for a gradient in the velocity dispersion, we found that the data of Falco et al. suggest the existence of an extremely massive object of about 1010 h-1 solar masses, whereas all updated measurements lead to a best fit characterized by a dark nucleus of 109 h-1 solar masses (click here to see the data and their interpretation: blue circles (Falco et al.), green circles (Tonry & Franx), red square (Spanish group), solid line (best global fit) and dashed line (best fit using exclusively the data of Falco et al.). The analysis is available at the astro-ph archive.